Secondary MX records

Why in the world would we want to complicate things further with a second MX record - there's a real good reason for this. If for whatever the reason, your email server is not able to accept mail, it is possible that you could lose some incoming email depending on the duration of the outage. Legitimate email servers -  that is to say ones not used for spamming - will retry any email that is undeliverable due to an outage.

Because email servers will retry delivery periodically, the duration or retry attempts that any sending servers will retry can vary. For short outages of less than a couple of days, you should not expect to lose any email. But you could. Very short durations of an hour or to are fairly safe, but you still don't know for sure.

This is where a secondary MX record comes in handy. In the event that your email server specified by your primary MX record becomes unavailable, a sending email server will try to look up, the next MX record, and the next and next until it finds someone who will accept the email.

Store and forward servers or Gateways

Email servers designated as secondary MX destinations are known as store and forward servers. That is to say, that they will store incoming email for an unreachable destination and forward the email to the server as soon as it becomes available again - retrying every X minutes for X number of days.

Now lets suppose that your ISP or any other service provider provides a store and forward service. Or perhaps you prefer to run your own on a different network or at a collocation. This server is setup to accept email for your domain, and continue to try to deliver for some period of time. The difference here is that you either know, or control the retry duration of that backup email server, so you have more control.

MX record priority

In your previous discussion of MX records, we mentioned that there are some additional parameters involved with MX records. Priority is an important parameter in the discussion of secondary MX records. Priority is what differentiates the primary email server from the secondary. The Primary MX record having a higher priority than the secondary - inversely specified with an integer - the lower the number, the higher the priority.

There is no set number that a primary must be set to. It must simply have a lower priority value than the secondary. That's all. It is common to see a primary with a priority of 10, and a secondary set to priority 20. A tertiary MX record might very well then be set to priority of 30, and so on.

But beware

There are drawbacks to running a store and forward server, and specifying one as a secondary MX record - Spammers! It is a common practice for spammers to ignore a primary MX record, and send their payload directly to a store and forward server specified by a secondary MX record. They do this in the hopes that they are getting past, or around any spam filtering you may have deployed. This creates a heavy load on store and forward servers, since in essence they are just like gateways without the filtering - they accept all email for a hosted domain, unaware if the recipient is a valid mailbox or not.

Anyone with a store and forward server or service will notice a large volume of spam originating from their store and forward service.