Configure Sendmail to Relay Emails through Gmail SMTP

Sendmail provides the option to send emails via the external SMTP servers. This tutorial will help you to configure Sendmail to relay email through external SMTP servers like Gmail, Amazon SES, Mailchimp, Mandrill etc. In this tutorial, we are using the Gmail SMTP server to relay emails from Sendmail server.

1. Create Auth File

First create directory for storing authentication files.

sudo mkdir /etc/mail/authinfo
sudo chmod 700 /etc/mail/authinfo

Now, create an auth file with any name like smtp-auth and add the following content. Replace the above email with your gmail or google apps email.

sudo vi /etc/mail/authinfo/smtp-auth

AuthInfo: "U:root" "I:[email protected]" "P:PASSWORD"

Now create a hash map file of above created auth file.

makemap hash /etc/mail/authinfo/smtp-auth < /etc/mail/authinfo/smtp-auth

2. Configure Sendmail with SMART_HOST

Now add following configuration lines into your sendmail.mc configuration file below "MAILER" definition.

 define(`SMART_HOST',`[smtp.gmail.com]')dnl
 define(`RELAY_MAILER_ARGS', `TCP $h 587')dnl
 define(`ESMTP_MAILER_ARGS', `TCP $h 587')dnl
 define(`confAUTH_OPTIONS', `A p')dnl
 TRUST_AUTH_MECH(`EXTERNAL DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
 define(`confAUTH_MECHANISMS', `EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
 FEATURE(`authinfo',`hash -o /etc/mail/authinfo/smtp-auth.db')dnl

In the next step we will need to re-build sendmail's configuration. To do that execute:

make -C /etc/mail

3. Verify Setup

After completing all the above configuration's, reload the sendmail service to apply changes.

sudo service sendmail reload

Now send a test email to verify everything is working fine.

echo "Test Email" | mail -s "Subject Here" [email protected]