Have any good suggestions for Mail server contingency planning?

ie. mail server relay

You typically have a second mail server with a higher MX record, then if the first fails to respond then the mail gets relayed to the second. The second server queues it until the first server comes back up then its relayed back to the primary server and is made available to your users. This way you loose any access to new mail comming in, but its not rejected when someone tries to send it to you, it just gets queued for longer.

Another possible solution would be to have two mail servers that have their mail boxes located on some sort of shared file system (say NFS) so that they actually deliver to the same place. This way if one went down then you'd still get access to your mail, clearly this then introduces another single point of failure - the NFS server.

A third possibilty, but probably the most complex would be to have a second server that is an exact replica of the first that when the first fails it takes over using the same IP address/hostname etc... this would be completely invisable to both incomming mail and your users.

cheers

Jamie...

Thanks for your suggestion! The first suggestion you already help me how to do it. But this suggestion the user also can't receive mail when the first server also down. And the second suggestion you also talk to me, but I afraid this suggestion have security issue. I heard NFS doesn't secure when it exposed in the Internet. And the third suggestion I also heard from my friend, but he also don't know how to do it. And how about the third suggestion? Have any secure issue or problem need to attention and what should I do if I follow this suggestion? Thanks a lot!!!

You said about NFS security issues - true but you don't need to expose the NFS mounts to the internet - just to the mail servers

cheers

Jamie...

Just remember this is linux, so the backup server will not be getting a lot of use, it's there just in case. The main server should have the ability to stay up hundreds of days straight.