Add MX records to your DNS like this:
You will also need to add DNS entries that correspond with the new server so that backup.yourdomain.com points to the IP address of your back-up mail server.
When the mail server at the IP address relating to mail.yourdomain.com is not available mail will be sent to backup.yourdomain.com
and then add the following line in /etc/mail/mailertable:
Next we need to add this to the mailertable database.
# makemap hash /etc/mail/mailertable.db < /etc/mail/mailertable And then append the following lines to the bottom of your access file. To:yourdomain.com RELAY Now you should have something like this: # by default we allow relaying from localhost... localhost.localdomain RELAY localhost RELAY 127.0.0.1 RELAY To:yourdomain.com RELAY now run: makemap hash /etc/mail/access.db < /etc/mail/access Now restart the sendmail service to make the changes complete Type: # service sendmail restart