Where your e-mails go when your main mail server decides it doesn't want to talk to you anymore... --Kazuhiko
Ooooh... I wasn't aware that this system worked. How well does it cope with, say, 90% uptime but twelve hour outages on the primary mx? It'd be kinda nice to have my own mail domain, but I was under the impression that all my mail would just bounce while my system was down. --Vitenka
Well, no. Should work fine, since the sender should keep retrying for a few days before giving up. Believe me, if the sender gave up after the first try, almost nothing would get through to several big mail systems I know of, mentioning no names Hotmail. -- Admiral
The idea is your DNS records contain a list of servers that accept mail for your domain, along with priorities. SMTP/DNS specs say mail goes to the highest priority (i.e. numerically lowest) server that's up. If none are up, mail is queued by the sender which retries for (typically) several days before giving up. The exact details of which servers are up or down don't matter, nor does it matter how long they're up or down for.
You want to get someone other than yourself to provide the low-priority servers, because if your network is down so are all your servers. Typically, the low-priority offsite servers are set up to hold incoming mail in a spool and keep trying to send it to a higher-priority server until one accepts it; that way it all eventually ends up somewhere your users can read it. You have to negotiate for spool space with whoever is providing your backups, based on how long you expect things to go down for at a time and how much mail you get in that time - from this point of view, expected outages matter.
Having read our downtime record, BlackCatNetworks? decided to charge us a tenner a year for the service.
Well, I have significantly more downtime because I take stuff down to play ComputerGames?. How much would you charge me to be secondary? (I could be tertiary for you, but I'm down more often than you are) Then again, with luck, my downtime will go away soon, once I get this 'nix set up. --Vitenka
Dunno. You're just one person - it's not like you're gonna get huge wodges of mail. And it's not like we're professional, and we almost know you :) Don't see how we could charge, really ;) - MoonShadow
It's only sensible to have a secondary MX if you have administrative control over it or can at least cause it to accept the same set of local parts as your primary MX. Otherwise the secondary has to construct a bounce for any misaddressed mail when it attempts to forward it to the primary, rather than just sending a 5xx SMTP response. This used to be a minor inconvenience; with large quantities of spam having bogus return paths it's more serious today. (Since senders retry, having backoff MX hosts adds less reliability than people seem to think; better therefore to do away with the extra complexity.)