Setting up backup mail exchange server with sendmail
Most systems admin is used to setting up the primary mail server, but many of them (like me) are not used to setup a backup mail server in a regular basis. This is my attempt to create a complete document on setting up a backup mail server and testing it for the dooms day. (Actually it in used more often than you can think of)
Lets say we have mail server for @example.com
Therefore there is a mail server called mail.example.com.
Mail and dns works hand in hand. DNS server tells where to deliver the email. MTA always looks for DNS entry to find the primary and secondary mail server incase the primary is not reachable. So the first thing we need to do it setup a DNS entry for the backup mail server