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Overview

1. How to Set up Sendmail to Allow Email Relay from a Specific IP?
2. Steps Needed
3. Important Considerations
4. Conclusion

How to Set up Sendmail to Allow Email Relay from a Specific IP?

Sendmail, a versatile and widely used Mail Transfer Agent (MTA), enables email communication between servers. A crucial feature of Sendmail is its ability to relay emails from specific IP addresses without requiring SMTP authentication. This functionality is often necessary for applications or systems that need to send emails securely and efficiently. In this guide, we will walk through the steps to configure Sendmail to allow email relays from specified IP addresses.

Email Relay involves forwarding email messages from one server to another. When configured, Sendmail can accept and forward emails originating from a trusted IP address without further authentication.

Prerequisites

• Root or Sudo Access: Administrative privileges are required for configuration.

• Backup Configuration Files: Always back up the Sendmail configuration files to prevent data loss:

sudo cp -r /etc/mail /etc/mail.bak

Steps Needed

Step 1: Edit the Access Database

The access database controls which systems can relay emails via Sendmail.

Open the Access File:

Edit the /etc/mail/access file using a text editor:

sudo vim /etc/mail/access

Add IP Address Entry:

Allow relaying for a specific IP address by adding:

Connect:XXX.XXX.X.XXX RELAY

Replace XXX.XXX.X.XXX with the desired IP address.

Allow Multiple IPs:

To allow relaying for multiple IPs, add a line for each IP in the same format:

Connect:192.168.1.100 RELAY
Connect:192.168.1.101 RELAY

Save and Exit:

Save the file and exit the editor.

Step 2: Rebuild the Access Database

Rebuild the access database to make the changes effective.

Run the following command to create a new access.db file:

sudo makemap hash /etc/mail/access < /etc/mail/access

Step 3: Edit the Main Configuration File

Open sendmail.mc:

Edit the /etc/mail/sendmail.mc file:

sudo vim /etc/mail/sendmail.mc

Uncomment Authentication Options:

Ensure the authentication line is not commented out. Modify:

dnl define(`confAUTH_OPTIONS’, `A p’)dnl

To:

define(`confAUTH_OPTIONS’, `A p’)dnl

Check for SMTP Authentication (optional):

If SMTP authentication is used, verify this line exists:

FEATURE(`authinfo’, `hash -o /etc/mail/authinfo/authinfo.db’)dnl

Save and Exit:

Save changes and close the file.

Step 4: Regenerate Configuration Files

Compile the changes into the sendmail.cf file.

Run the following command:

sudo make -C /etc/mail

Step 5: Restart the Sendmail Service

Restart the Sendmail service to apply the new configuration.

Restart Command:

sudo service sendmail restart

Verify Service Status (optional):

Check if the service is running correctly:

sudo service sendmail status

Step 6: Test Email Relay

To confirm the setup, send a test email from the configured IP address.

Send Test Email:

Run the following command:

echo “Test email from $(hostname -f)” | mail -s “Test Email” recipient@example.com

Check Logs:

Review mail logs for success or error messages:

sudo tail /var/log/maillog

Important Considerations

1. Open relaying can expose the server to spam abuse. Restrict relaying strictly to trusted IP addresses.

2. Regularly review /var/log/maillog for suspicious activity or unauthorized relay attempts.

3. Combining IP-based relay with SMTP authentication adds an extra layer of security.

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Conclusion

Configuring Sendmail to allow email relays from specific IP addresses is a powerful feature for streamlining email workflows. By following these steps and adhering to security best practices, we can enable trusted applications or systems to send emails while maintaining a secure and reliable server.