This is a quick guide on how to use sendmail as a backup MX server. If you want to use qmail as a backup MX/backup mail server, please see our Qmail as backup MX guide here.
1. Installing sendmail
If you do not have sendmail installed, you can install it using your OS package manager. In this example, we use yum on CentOS:
yum install sendmail
2. Configuring the sendmail service
First off, we want to make sure it auto-starts upon reboot:
chkconfig sendmail on
By default, sendmail will only listen on localhost; for this example we will set it to listen on all interfaces instead:
1. Open /etc/mail/sendmail.cf
2. Find the line starting with DaemonPortOptions and then modify it so it looks like this:
3. Configuring sendmail to run as backup MX/backup mail server
First, we will need to modify the sendmail access file.
1. Open the access file:
2. For every domain that you want to allow this sendmail instance to serve as a backup mail server for, add the following line (replace DOMAIN.com with your real domain name):
3. Next, you will need to specify which mail route this sendmail instance will attempt to deliver mail to (replace DOMAIN.com with your real domain name):
4. Finally, we will recreate the database files:
makemap hash /etc/mail/mailertable.db < /etc/mail/mailertablemakemap hash /etc/mail/access.db < /etc/mail/access
5. Make sure to restart sendmail so that our new configuration is taken in to effect:
4. Update DNS
Now that you have configured your sendmail service to act as a backup mail server, you will need to update your DNS so that other mail servers know that you have a backup MX.
If you have not already, create a DNS A record for the backup mail server, eg: mail-backup.DOMAIN.com
Next, create a new MX record that points to your DNS name for the backup mail server with a higher priority number than the primary mail server. Remember, the higher the priority number, the less precedence it has, eg in order of preference:
DOMAIN.com MX 10 mail.DOMAIN.com
DOMAIN.com MX 50 mail-backup.DOMAIN.com
Now that you have configured sendmail and your DNS, you should now have a backup MX configured ! If you have any issues, questions or concerns, please comment and I will assist where I can.