Configure sendmail to use smtp relay

smtp_relay.txt

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:user@gmail.com" "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" recipient@example.com