Two mx records for two different mail servers in the same domain?

Question

I have a domain named example.com. I want to set two mail servers on this domain. The main purpose to set these mail servers is to have two different mail server, as if we have two different domains => so it is not one main server and the other one for backup.

An MX record for domain example.com is already existing (all mails sent to user@example.com are delivered to smtp.example.com), so we have

@   3600    IN  MX  1   smtp.example.com

The A records are set like this:

@   600 IN  A   12.12.12.12
smtp    3600    IN  A   13.13.13.13
relay   3600    IN  A   14.14.14.14</pre>

Can we add new MX records for the second mail server?

For example, we set up a new subdomain relay.example.com that points to our new mail server (at ip 14.14.14.14). So that all mail that are sent to user@relay.example.com will be sent to relay.example.com.

Answer 1

This should do it:

@     3600 IN A    12.12.12.12    ; irrelevant for mail routing

@     3600 IN MX 1 smtp.example.com.
smtp  3600 IN A    13.13.13.13

relay 3600 IN MX 1 relay.example.com.
relay 3600 IN A    14.14.14.14

Notes:

• The last MX record is not strictly required (according to the standards, if there is no MX record for a domain then its A record is tried as a fallback - but some MTAs might not like that)
• Don't forget the terminating dot at then end of fully qualified records in your zonefile!

To avoid confusion, here a clarifying variant that would send all mail targeted at @subdom.example.com to the MTA relay.example.com:

subdom 3600 IN MX 1 relay.example.com.
relay  3600 IN A    14.14.14.14

Answer 2

Of course, yes. A domain can have as many MX records (and thus, mail servers) as you wish. You can even set up priorities between them. For example, the MX records of the domain gmail.com are (you can get this a command host -t MX -v gmail.com):

gmail.com.              3547    IN      MX      5 gmail-smtp-in.l.google.com.
gmail.com.              3547    IN      MX      10 alt1.gmail-smtp-in.l.google.com.
gmail.com.              3547    IN      MX      20 alt2.gmail-smtp-in.l.google.com.
gmail.com.              3547    IN      MX      30 alt3.gmail-smtp-in.l.google.com.
gmail.com.              3547    IN      MX      40 alt4.gmail-smtp-in.l.google.com.

The outer mail servers trying to deliver mail to your domain are expected to follow the priorities in ascending order. Same priorities means redundancy (although it isn't really good on my experience).

In this example, the first try will be gmail-smtp-in.l.google.com., the second alt.gmail-smtp-in.l.google.com, and so on. The priorities are in before the last column.

Some similar can you set up in your domain. In your place I did it as a secondary mail server, for backup purposes if the first isn't really working.