Learn How to Send Emails via SMTP [Send Mail Using Your Server Provider & Development Environment]

Introduction to Simple Mail Transfer Protocol (SMTP)

What is SMTP

Simple Mail Transfer Protocol (SMTP) is a protocol used for sending emails from a client to a server or between servers themselves. It's the standard protocol for sending emails on the internet, ensuring that your message is relayed from your email client to the recipient's email server.

At its core, SMTP handles the sending part of the email communication, working closely with protocols like POP (Post Office Protocol) or IMAP (Internet Message Access Protocol), which manage the receiving aspect.

Types of SMTP Servers

There are primarily three types of SMTP servers, each serving a different purpose in the email sending process:

Standard SMTP Servers

These are the basic servers that directly handle the sending of emails. They are configured to use SMTP to send messages to the recipient's mail server.

SMTP Relay Services

These servers are used to relay emails from one server to another, often used when the sender is in a different network from the receiver.

SMTP relay servers are crucial for overcoming restrictions that ISPs (Internet Service Providers) place on outgoing emails to prevent spam.

SMTP Email Servers

Specialized in email services, these servers, like Gmail SMTP, Outlook SMTP, and Yahoo Mail SMTP, offer enhanced security and features for sending emails. They require authentication, ensuring that only authorized users can send emails.

SMTP Infrastructure

The SMTP infrastructure consists of several key components working together to ensure the efficient and secure transfer of email messages from senders to recipients. This infrastructure includes:

SMTP Servers

These servers are the workhorses of the SMTP infrastructure, handling the sending, receiving, and forwarding of email messages.
They operate by communicating with each other through SMTP commands and responses to transfer messages from the sender's email client to the recipient's mail server.

Mail Transfer Agents (MTAs)

MTAs are software applications residing on SMTP servers, responsible for transferring email messages from one server to another.
They play a crucial role in the email delivery process, ensuring that emails are correctly routed to their destination servers.

SMTP Relay Services

To facilitate the delivery of emails across different networks and circumvent ISP restrictions, SMTP relay services are employed.
These services act as intermediaries, receiving emails from senders and forwarding them to the appropriate destination servers. They are essential for businesses and organizations that send large volumes of email.

Mail Delivery Agents (MDAs)

Once an email reaches the recipient's server, MDAs take over.
These agents are responsible for delivering the email from the server to the recipient's mailbox, making it accessible through email clients or webmail interfaces.

Mail User Agents (MUAs)

MUAs are the client-side applications that users interact with to compose, send, and receive emails.
Examples include webmail clients like Gmail and desktop applications like Microsoft Outlook. MUAs use SMTP to send outgoing emails and protocols like IMAP or POP3 to retrieve incoming messages.

Security Measures

SMTP Authentication: It verifies the sender's identity, reducing the chances of email spoofing and phishing attacks.
TLS Encryption: Ensures that emails are transmitted securely, protecting sensitive information from interception.
Impact: These measures significantly lower the risk of data breaches, with encryption helping to prevent over 58% of unauthorized email access attempts.

How Does SMTP Work

Simple Mail Transfer Protocol (SMTP) is the standard communication protocol for sending emails across the internet. Its operation is foundational to email communication, ensuring that messages are properly routed from senders to recipients.

Let's break down how SMTP works, combining an overview with specific details:

Establishing the Process

SMTP functions through a client-server model, where an email client (the sender) interacts with an SMTP server to send email messages.

This process involves several steps, beginning with the sender composing an email and hitting send. Here’s a closer look:

Connection: The email client connects to the SMTP server, using the SMTP protocol. This is often done over port 25, which is specifically reserved for SMTP communication.
Communication: Once connected, the client and server communicate using a series of text-based commands and responses. These commands include HELO (or EHLO for Extended SMTP), MAIL FROM, RCPT TO, and DATA.

The SMTP Conversation

HELO/EHLO: The email client identifies itself to the server, starting the SMTP session.
MAIL FROM: Specifies the sender's email address.
RCPT TO: Indicates the recipient's email address(es).
DATA: Begins the message transmission, including the email's body and subject.

After the DATA command, the email client sends the actual message content, followed by a single period (.) on a line by itself to signify the end of the message.

Routing and Delivery

SMTP Servers and Relays: The SMTP server may need to relay the email through one or more other SMTP servers to reach the recipient's email server. This is particularly true if the sender and recipient use different email services (e.g., sending from a Gmail account to a Yahoo account).
Final Delivery: Once the email reaches the recipient's SMTP server, it is handed off to a Mail Delivery Agent (MDA) which places the email in the recipient's mailbox.

Why SMTP is Effective

Reliability: SMTP includes mechanisms for queueing emails, attempting re-sends if the initial delivery fails, and providing feedback via bounce messages if an email cannot be delivered.
Flexibility: It can be used in conjunction with other protocols like IMAP or POP3, which handle the retrieval of messages from mailboxes, offering a complete solution for email transmission and retrieval.
Security Enhancements: Modern SMTP servers often incorporate extensions like STARTTLS to encrypt email transmissions, enhancing privacy and security.

Practical Example

Imagine sending an email from a Gmail account to an Outlook account:

Your email client connects to Gmail’s SMTP server and initiates the SMTP handshake (HELO/EHLO).
The Gmail SMTP server receives the email and looks up the MX (Mail Exchange) records for the recipient’s domain (Outlook) to find the correct SMTP server to which the email should be delivered.
Gmail’s SMTP server sends the email to Outlook’s SMTP server. If the direct path is unavailable, it may route through intermediate SMTP servers.
Upon arrival at Outlook’s SMTP server, the email is passed to an MDA, which places it in the recipient’s Outlook mailbox.

SMTP's design, rooted in simplicity and efficiency, has enabled it to remain the backbone of email communication, adapting to the internet's growth and the increasing demands for secure and reliable email transmission.

The Role of SMTP Servers in Email Delivery

SMTP servers are pivotal in the email delivery process, acting as the digital post offices of the internet. Their primary role is to send, receive, and relay emails between senders and recipients.

When you dispatch an email, your email client interacts with an SMTP server. This server then decides the pathway to deliver the message to the recipient's email server, using the domain name in the email address as a guide

Here's how they contribute to email delivery:

Routing: SMTP servers use domain names to route emails to the correct destination. They consult DNS servers to find the Mail Exchange (MX) records for the recipient's domain, which tell them where to send the email.
Queuing and Retry: If the destination server is unreachable, SMTP servers queue emails and periodically retry sending them. This ensures delivery attempts continue even in the face of temporary network issues.
Handling Bounces: If an email can't be delivered (e.g., due to a nonexistent address), SMTP servers handle the bounce. They send a notification back to the sender, informing them of the delivery failure.

How to Send an Email Using SMTP

Sending an email using Simple Mail Transfer Protocol (SMTP) involves several steps that can be accomplished programmatically through various programming languages or via SMTP clients.

Below, we break down the process into understandable steps, focusing on a generic approach that can be adapted to specific programming languages or SMTP service providers.

Step 1: Choose an SMTP Service Provider

Options: You can use your hosting provider's SMTP server, a dedicated email service (like SendGrid, Mailgun, or Amazon SES), or even the SMTP server of a free email provider (like Gmail, Outlook, or Yahoo Mail).
Credentials: Obtain the SMTP server details, including the server address, port, and authentication credentials (username and password).

Step 2: Configure SMTP Server Settings

Server Address: This is the SMTP server's domain name or IP address (e.g., smtp.gmail.com for Gmail).
Port: Common SMTP ports are 25 (unsecured), 587 (secured using STARTTLS), and 465 (secured using SSL/TLS).
Authentication: Most SMTP servers require authentication. Use the credentials provided by your SMTP service.

SMTP Server Details of Popular Email Services

Gmail SMTP Authentication

SMTP Server Address: smtp.gmail.com
Port: 587 for TLS/STARTTLS or 465 for SSL
Username: Your full Gmail address (e.g., yourname@gmail.com)
Password: Your Gmail password (or an App password if Two-Factor Authentication is enabled)

Outlook SMTP Authentication

SMTP Server Address: smtp-mail.outlook.com
Port: 587
Encryption Method: STARTTLS
Username: Your full Outlook email address (e.g., yourname@outlook.com)
Password: Your Outlook password

Yahoo Mail SMTP Authentication

SMTP Server Address: smtp.mail.yahoo.com
Port: 465 (SSL required)
Username: Your full Yahoo email address (e.g., yourname@yahoo.com)
Password: Your Yahoo password

SMTP Server Details for a Development Environment

When sending emails programmatically, you authenticate with the SMTP server using similar details but within your code. Here's a Python example using smtplib for Gmail:

import smtplib from email.mime.text import MIMEText from email.mime.multipart import MIMEMultipart # Gmail SMTP server configuration smtp_server = "smtp.gmail.com" port = 587 # For starttls sender_email = "your.email@gmail.com" password = "yourapppassword" # Your Gmail app password or less secure app password receiver_email = "recipient@example.com" # Create the email message (MIME) message = MIMEMultipart("alternative") message["Subject"] = "Test Email from Gmail" message["From"] = sender_email message["To"] = receiver_email # Create the plain-text and HTML version of your message text = """\ Hi, This is a test email sent from Python using Gmail SMTP.""" html = """\ <html> <body> Hi, This is a test email sent from Python using Gmail SMTP. </body> </html> """ # Attach both plain-text and HTML versions to the message part1 = MIMEText(text, "plain") part2 = MIMEText(html, "html") message.attach(part1) message.attach(part2) # Send the email try: server = smtplib.SMTP(smtp_server, port) server.starttls() # Secure the connection server.login(sender_email, password) server.sendmail(sender_email, receiver_email, message.as_string()) print("Email sent successfully!") except Exception as e: print(f"Error sending email: {e}") finally: server.quit()

Step 3: Compose Your Email

From: The sender's email address.
To: The recipient's email address.
Subject: The subject line of the email.
Body: The main content of the email. It can be plain text or HTML for a styled message.

Step 4: Configure SMTP for Sending Email

For Server Providers:

Access Your Hosting/Email Provider's Dashboard: Log in to the control panel provided by your hosting or email service.
Locate SMTP Settings: Find the section for email settings or SMTP configurations. This is typically under account settings or email management.
Enter SMTP Details: Input the SMTP server address, port, username, and password as specified by your service provider. If you're using a common email service like Gmail or Outlook, they provide this information in their help sections.
Test Configuration: Most services offer a way to test the SMTP settings by sending a test email. Use this feature to ensure your configuration is correct.

For Development Environments:

Choose a Programming Language and Library: Depending on your application's backend language, select an SMTP library or module. For example, use smtplib for Python, javax.mail for Java, or System.Net.Mail for .NET.
Write the SMTP Code: Create a script or function that defines the SMTP server details, authenticates with the server, composes the email, and sends it.

Here's a brief example in Python using smtplib:

import smtplib from email.mime.text import MIMEText from email.mime.multipart import MIMEMultipart # SMTP server configuration smtp_server = "smtp.example.com" port = 587 # For starttls sender_email = "your_email@example.com" password = "your_password" # Create a secure SSL context context = smtplib.SMTP(smtp_server, port) context.starttls() # Upgrade the connection to secure # Log in to the server context.login(sender_email, password) # Email details receiver_email = "receiver@example.com" message = MIMEMultipart("alternative") message["Subject"] = "Test Email" message["From"] = sender_email message["To"] = receiver_email# Create the plain-text and HTML version of your message text = """\ Hi, How are you? Real Python has many great tutorials. www.realpython.com""" html = """\ <html> <body> Hi, How are you? <a href="www.realpython.com">Real Python</a> has many great tutorials. </body> </html> """ # Turn these into plain/html MIMEText objects part1 = MIMEText(text, "plain") part2 = MIMEText(html, "html") # Add HTML/plain-text parts to MIMEMultipart message # The email client will try to render the last part first message.attach(part1) message.attach(part2) # Send email context.sendmail(sender_email, receiver_email, message.as_string()) # Close the connection context.quit()

Step 5: Send the Email

Execute your script. The SMTP client will connect to the SMTP server using the specified server details and authentication credentials.
The server processes the request, and if everything is correct (e.g., valid credentials and properly formatted email), it will relay the email to the recipient's email server.

Step 6: Handle Responses

Success: You may receive a confirmation from the SMTP server that the email was accepted for delivery.
Error Handling: Be prepared to handle errors, such as authentication failures, missing information, or server issues. Your SMTP client or library should provide mechanisms for catching these errors.

How to Set Up an SMTP Server on Windows/Linux

Setting up an SMTP server on Windows or Linux systems, as well as configuring one for web applications, involves a series of steps that require administrative rights and a basic understanding of your network and server environment.

Below, we'll go through the processes for both environments.

SMTP Server on Windows

For Windows Server (e.g., Windows Server 2019):

1. Install the SMTP Server Feature:

Open the Server Manager.
Go to "Manage" -> "Add Roles and Features".
Navigate through the wizard and select "SMTP Server" under the "Features" section.

2. Configure SMTP Service:

After installation, open the Internet Information Services (IIS) 6.0 Manager (note: SMTP is managed through IIS 6.0 Manager even on newer servers).
Right-click on "SMTP Virtual Server #1" -> "Properties".
Configure general settings (e.g., IP address, port (usually port 25), and limits).

3. Set Up SMTP Email:

Under "Access" tab, configure connection and relay restrictions.
Under "Delivery" tab, configure outbound connections, advanced delivery options including smart host settings if you are relaying emails through an external SMTP server.

4. Secure Your SMTP Server:

It's crucial to restrict relay to prevent spam. Only allow your server IP or specific IP addresses to relay emails.
Consider enabling TLS for secure email transmission.

5. Test Your SMTP Server:

Use telnet or PowerShell (Send-MailMessage cmdlet) to send a test email and verify that your SMTP server is working correctly.

SMTP Server on Linux

For Linux (e.g., using Postfix on Ubuntu):

1. Install Postfix:

Run sudo apt-get update followed by sudo apt-get install postfix.
During installation, you will be prompted to choose a configuration. For most users, "Internet Site" is appropriate.

2. Configure Postfix:

Edit the main configuration file (/etc/postfix/main.cf) to set up domain, IP, and other settings.
Specify myhostname, mydomain, myorigin, and relay host (relayhost) if applicable.

3. Control Access and Relay:

Edit /etc/postfix/main.cf to set up mynetworks (IP addresses allowed to send emails) and adjust relay controls.

4. Enable and Start Postfix:

Run sudo systemctl enable postfix and sudo systemctl start postfix to enable and start Postfix automatically.

5. Test Your SMTP Server:

Use commands like telnet or sendmail for basic email sending tests to ensure the SMTP server is operational.

Send Emails with SMTP in Python

Python provides the smtplib library, which defines an SMTP client session object that can be used to send mail to any Internet machine with an SMTP or ESMTP listener daemon.

import smtplib from email.mime.text import MIMEText from email.mime.multipart import MIMEMultipart # SMTP server configuration smtp_server = "smtp.example.com" port = 587 # For starttls sender_email = "your-email@example.com" password = "your-password" receiver_email = "receiver@example.com" # Create the email message message = MIMEMultipart("alternative") message["Subject"] = "Test Email" message["From"] = sender_email message["To"] = receiver_email # Create the plain-text and HTML version of your message text = """\ Hi, This is a test email sent from Python.""" html = """\ <html> <body> Hi, This is a test email sent from Python. </body> </html> """ # Attach parts into message container part1 = MIMEText(text, "plain") part2 = MIMEText(html, "html") message.attach(part1) message.attach(part2) # Send the email try: server = smtplib.SMTP(smtp_server,port) server.starttls() # Secure the connection server.login(sender_email, password) server.sendmail(sender_email, receiver_email, message.as_string()) finally: server.quit()

Send Emails with SMTP in Java

JavaMail API can be used to send emails via SMTP. You need to include the JavaMail dependency in your project to use it.

import javax.mail.*; import javax.mail.internet.*; import java.util.Properties; public class SendEmail { public static void main(String[] args) { String host = "smtp.example.com"; String user = "your-email@example.com"; String pass = "your-password"; String to = "receiver@example.com"; String subject = "Test Email"; String messageText = "This is a test email sent from Java."; Properties props = System.getProperties(); props.put("mail.smtp.host", host); props.put("mail.smtp.user", user); props.put("mail.smtp.password", pass); props.put("mail.smtp.port", "587"); props.put("mail.smtp.auth", "true"); props.put("mail.smtp.starttls.enable", "true"); Session session = Session.getDefaultInstance(props, null); MimeMessage message = new MimeMessage(session); try { message.setFrom(new InternetAddress(user)); message.addRecipient(Message.RecipientType.TO, new InternetAddress(to)); message.setSubject(subject); message.setText(messageText); Transport transport = session.getTransport("smtp"); transport.connect(host, user, pass); transport.sendMessage(message, message.getAllRecipients()); transport.close(); } catch (MessagingException me) { me.printStackTrace(); } } }

Send Emails with SMTP in Node.js

Node.js allows sending emails using SMTP with the help of the nodemailer module. First, install nodemailer using npm:

npm install nodemailer

Then, use the following code to send an email:

const nodemailer = require('nodemailer'); let transporter = nodemailer.createTransport({ host: "smtp.example.com", port: 587, secure: false, // true for 465, false for other ports auth: { user: "your-email@example.com", pass: "your-password", }, }); let mailOptions = { from: '"Sender Name" <your-email@example.com>', to: "receiver@example.com", subject: "Test Email", text: "This is a test email sent from Node.js.", html: "This is a test email sent from Node.js.", }; transporter.sendMail(mailOptions, (error, info) => { if (error) { return console.log(error); } console.log('Message sent: %s', info.messageId); });

How to Integrate SMTP with WordPress

Integrating SMTP (Simple Mail Transfer Protocol) with WordPress ensures that your website's emails are reliably sent through a professional mail server, significantly reducing the chances of your emails being marked as spam.

Here’s a step-by-step guide on how to set up SMTP in WordPress:

Step 1: Choose an SMTP Plugin

WordPress does not natively support SMTP email configuration, so you will need to use a plugin. There are several SMTP plugins available, but one of the most popular and user-friendly is "WP Mail SMTP" by WPForms.

Go to your WordPress dashboard.
Navigate to "Plugins" > "Add New".
Search for "WP Mail SMTP".
Click "Install Now" on the plugin, then activate it.

Step 2: Configure the SMTP Plugin

After activating the plugin, you need to configure it with your SMTP service details.

In your WordPress dashboard, go to "WP Mail SMTP" > "Settings".
Here, you will see the "From Email" and "From Name" options. The "From Email" should be the email address you want to send emails from, and it's recommended this is a valid address on your domain to improve deliverability.
Check the box that says "Set the return-path to match the From Email" to ensure bounce messages are sent to the From Email address.

Step 3: Choose Your SMTP Mailer

WP Mail SMTP allows you to choose from several SMTP services, including:

Default SMTP
Sendinblue SMTP
Mailgun SMTP
SendGrid SMTP
Gmail SMTP (for Gmail or G Suite accounts)
Microsoft SMTP (Outlook.com and Office 365)
Amazon SES SMTP
Other SMTP

Select the mailer that best suits your needs. If you have an SMTP server already, choose “Other SMTP”.

Step 4: Enter Your SMTP Settings

If you selected "Other SMTP", you would need to enter your SMTP server details. These details typically include:

SMTP Host: The address of your SMTP server.
SMTP Port: Common ports include 587 (recommended for TLS encryption) and 465 (for SSL encryption).
Encryption: Choose between SSL and TLS encryption. TLS is recommended if supported by your SMTP server.
Authentication: Enable authentication and enter the SMTP username and password provided by your SMTP service or hosting provider.

For other mailers, you will be prompted to enter API keys or OAuth credentials, which you can get from your email service provider's dashboard.

Step 5: Test Your SMTP Settings

WP Mail SMTP provides an option to send a test email.

Navigate to "WP Mail SMTP" > "Email Test" in your WordPress dashboard.
Enter an email address where you want to receive the test email and click "Send Email".
You should receive the test email, confirming that your SMTP settings are correctly configured.

Step 6: Save Your Settings

After successfully sending a test email, ensure all your settings are saved. Your WordPress site is now configured to send emails using SMTP, improving email deliverability.

Top 5 SMTP Services

1. SendGrid

Known for its robust delivery system, SendGrid offers detailed analytics, a flexible API, and strong deliverability. It's suitable for businesses of all sizes, providing both transactional and marketing email services.

2. Mailgun

Favored by developers for its powerful APIs, Mailgun specializes in transactional emails. It offers detailed analytics, email validation services, and flexible webhook options for real-time data.

3. Amazon Simple Email Service (SES)

Amazon SES is a cost-effective, scalable solution for sending transactional and marketing emails. It integrates well with other AWS services, making it a popular choice for businesses already using AWS.

4. Postmark

Postmark focuses on delivering transactional emails with high speed and precision. It's known for its straightforward pricing, excellent customer support, and detailed tracking capabilities.

5. SMTP.com

One of the oldest SMTP service providers, SMTP.com is renowned for its reliability and ease of integration. It offers high deliverability, comprehensive reporting, and a strong reputation for managing email campaigns.

Comparison: Free vs. Paid SMTP Services

Email Sending Limits when Using Free SMTP Server

SMTP vs. IMAP vs. POP3: Understanding Different Mail Transfer Agent

SMTP (Simple Mail Transfer Protocol), IMAP (Internet Message Access Protocol), and POP3 (Post Office Protocol version 3) are fundamental to email delivery and retrieval.

SMTP acts as the outgoing mail server, facilitating email sending. IMAP and POP3 are used for retrieving emails from the server, with key differences in how they manage emails and user interaction.

Feature	SMTP	IMAP	POP3
Primary Function	Sending emails	Retrieving and managing emails	Retrieving emails
Server Type	Outgoing mail server	Incoming mail server	Incoming mail server
Usage	Functions as the mail submission agent	Allows users to view and organize mails on the server	Downloads mails to the mail client and usually deletes them from the server
Key Characteristics	- Routes emails to recipient's email server - Uses smtp email server address for email routing	- Emails are stored on the server - Synchronizes email across multiple devices	- Emails are downloaded to a single device - Ideal for offline email access
Flexibility	- Essential for email sending - Configured in the mail client alongside IMAP or POP3	- High, due to server-side email management and multi-device access	- Limited, best for accessing email from one location
Data Security	- Relies on encryption for secure email transmission	- Keeps emails on the server, reducing data loss risk	- Downloads emails, posing a risk if the device is compromised
Typical Use Case	- Universal for sending emails across the internet	- Preferred by users who access email from multiple devices	- Suited for users who access email from a single device and need to conserve server space

Troubleshooting Common SMTP Errors

Troubleshooting SMTP errors is crucial for maintaining the flow of email communication. These errors can range from temporary issues with email delivery to authentication problems or configuration errors on the server.

Here are common SMTP errors and tips on how to troubleshoot them:

1. Authentication Errors (e.g., "535 Authentication Failed")

Verify Credentials: Ensure that the username and password are correct. Remember, some SMTP servers require the full email address as the username.
Check Authentication Method: Some SMTP servers may require a specific authentication method (e.g., STARTTLS). Ensure your email client or application is configured accordingly.

2. Connection Timed Out (e.g., "Timeout Waiting for Response")

Check Network Connection: Ensure the server hosting the SMTP service is accessible over the internet. Use tools like ping or traceroute to diagnose connectivity issues.
Verify SMTP Port and Address: Ensure you're connecting to the correct SMTP port (commonly 25, 465 for SSL, 587 for STARTTLS) and the server address is correct.

3. Server Does Not Support Encryption (e.g., "SSL/TLS Required")

Adjust Encryption Settings: If the server requires a secure connection, ensure your email client or application is set to use SSL/TLS. Conversely, if the server does not support SSL/TLS, adjust your settings to connect without encryption.

4. Email Address Not Found (e.g., "550 5.1.1 User Unknown")

Verify Recipient Email: Check the recipient's email address for typos or errors. Ensure the email address exists and is capable of receiving emails.
Check Email Forwarding Rules: If the recipient address is forwarding emails to another address, ensure that the forwarding rules are correctly configured.

5. Insufficient Permissions (e.g., "553 5.7.1")

Check Sender Address: Some SMTP servers require that the sender's email address matches the authenticated user. Verify that the sender's address is correct and allowed by the SMTP server.
Review Server Policies: Some servers have strict policies regarding who can send emails. Review these policies or contact the server administrator for guidance.

6. Size Limit Exceeded (e.g., "552 5.3.4 Message size exceeds fixed limit")

Reduce Email Size: Check the size of the email, including attachments. If it exceeds the server's limit, try reducing the attachment size or compressing files.
Server Configuration: If you're the server administrator, consider adjusting the maximum allowed message size in the server settings.

7. Cannot Relay (e.g., "554 5.7.1 Relay Access Denied")

Verify Recipient Domain: Ensure you're sending to a domain that your SMTP server is configured to relay messages to. Some servers restrict relaying to specific domains.
Check IP Whitelisting: If your SMTP server uses IP whitelisting for relaying, ensure your IP address is included in the whitelist.

‍