How to use sendmail as a backup MX/backup mail server

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:

DaemonPortOptions=Port=smtp,Addr=, Name=MTA


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:

vim /etc/mail/access

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):

To:DOMAIN.com                            RELAY

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):

DOMAIN.com smtp:mail.DOMAIN.com

4. Finally, we will recreate the database files:

makemap hash /etc/mail/mailertable.db < /etc/mail/mailertable
makemap hash /etc/mail/access.db < /etc/mail/access

5. Make sure to restart sendmail so that our new configuration is taken in to effect:

/etc/init.d/sendmail restart


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.



HOWTO qmail Store and Forward Backup Mailserver (backup MX)

This quick guide will explain how to setup qmail as a backup (store & forward) mailserver for a specific domain. In the instance of the primary mailserver going down, the source mailserver should failover to your next MX record, depending on it's priority. In this case, the next MX record will be this mailserver that we are configuring.

The backup mailserver will then store the mail received and will attempt to forward it on to the primary mailserver until it is accepting mail again..


First, we need to tell the backup mailserver to accept mail for the domain we are adding. We add this domain to /var/qmail/control/rcpthosts:

# vim /var/qmail/control/rcpthosts


Next, we need to tell qMail who the primary MX record is for this domain that we're storing & forwarding mail for. We add this information to /var/qmail/control/smtproutes:

# vim /var/qmail/control/smtproutes


And this information is added in the following format:



Once your configuration is complete, you need to reload qMail's configuration:

# /etc/init.d/qmail reload


Don't forget to add your new backup server's MX record to your DNS, with a higher number (lower priority) than your primary mailserver, for example:

domainName.com MX 10 primarymailserver.domainName.com
domainName.com MX 50 backupmailserver.domainName.com


And you're done!

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