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I will be setting up an Exchange email server at work soon but one thing I would like to know is what would happen if the mail server is down. Where would incoming messages go?

I understand that they will probably get lost, but is there a service that I can add as another MX record which, when the main exchange server goes down, it sends email to that and then when my server is back online, it sends them?

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Thanks for your help guys, I will try it in a virtual machine. Basically turning the server off and seeing if emails are still sent to it after a max of about 2 days. – Dean Perry Aug 7 '11 at 9:25
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3 Answers

up vote 3 down vote accepted

You could setup any number of failover mechanisms but in all reality, 99% of all email servers will simply queue the email (usually for a maximum of 48 hours) and continue to retry sending the email until your server is available again.

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OK so are secondary/backup MX services not really worth it then? – Dean Perry Aug 6 '11 at 22:38
its worth it in the sense that if you have an outage for more than 48 hours then you wont lose mail and other queues wont get backed up, its also not safe to rely on other servers holding mail for up to 48 hours, but rather having somewhere accepting the mail and forwarding it back to your exchange when its back online – anthonysomerset Aug 6 '11 at 22:42
I would take a look at mimecast, messagelabs, or postini. – SLY Aug 6 '11 at 22:53
What about Dyn's MX backup - dyn.com/email/dyn-email-backup-mx – Dean Perry Aug 6 '11 at 22:56
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Using third parties for MX backup can be problematic, as differences in configuration between your and their servers could result in a backdoor for spam (e.g., if they accepting everything, and you accept everything from them), bouncing of messages that should have been accepted, or not bouncing messages at all. For Dyn in particular, you probably ought to read Things Not To Do with Dyn Email Backup MX to see if any of those are incompatible with how you operate your primary. – Kanji Aug 7 '11 at 3:11
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up vote 1 down vote

RFC 5321 explains in detail how sending and receiving emails works.

Two important things in it I'll mention here

1) Mail that can't be delivered for whatever (temporary) reason MUST be queued and retried at a later time. Here the citation

Retries continue until the message is transmitted or the sender gives
up; the give-up time generally needs to be at least 4-5 days.  It MAY
be appropriate to set a shorter maximum number of retries for non-
delivery notifications and equivalent error messages than for
standard messages.  The parameters to the retry algorithm MUST be
configurable.

2) Mail that can't be delivered at all MUST be indicated to the sender. Here the citation:

If an SMTP server has accepted the task of relaying the mail and
later finds that the destination is incorrect or that the mail cannot
be delivered for some other reason, then it MUST construct an
"undeliverable mail" notification message and send it to the
originator of the undeliverable mail (as indicated by the reverse-
path).

So if all mail servers act accordingly to the standard there will be no mail loss at all. But it is said that there are some few mail servers that don't follow the rules. In this case it shouldn't be your responsibility to accept their mails in all cases only because they have crappy servers.

I would recommend to have a "cold-standby" mail server which you boot up only in case of a long Exchange down-time.

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I see, the only real reason the server would go down is really internet or power problems, which are usually fixed in the same day anyway. – Dean Perry Aug 7 '11 at 9:23
This is why SMTP was designed so failsafe. It was invented at a time where computers were EXPENSIVE and rare, and where you hadn't Internet connection and power all the time. Only nowadays everybody expects a 24/7-no-downtime service. – mailq Aug 7 '11 at 9:31
ah I see, thanks! :) – Dean Perry Aug 7 '11 at 9:32
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up vote 0 down vote

As long a the server isn't down for too long more than (two or three days) almost all your mail will arrive. E-mail works on a store and forward basis and the upstream servers will retry periodically. I believe the standard states they should retry for four days or so.

The good news is most spam is delivered by servers which try only once, so you won't have to filter a huge backlog of spam.

Setup a good alerting system and a UPS and you should be good for most cases. My connection to my ISP went down during a storm while I was on vacation and I was able to resolve the situation in a couple of days.

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