Here's the situation:

Server 1 = Main server
Server 2 = Backup server

1. Whenever Server 1 is down, all incoming & outgoing e-mail traffic will go to Server 2 by setting MX records on the DNS server.

2. When Server 2 receives incoming mail, it will deliver them to appropriate POP3 mailboxes. This will enable our users to still get their e-mail while Server 1 is down. (All the e-mail backup providers I've looked at keep them in some holding area until Server 1 is up and running again).

3. Server 2 needs to periodically check if Server 1 is up. If it is, then it needs to send any e-mails that are in the POP3 mailboxes to Server 1.

I've got #1 and #2 taken care of. The hang up is writing a cron job to perform #3.

Does anyone know how to do step 3? How about a "how-to" website? Actually, I'd really like to find software that does it; any recommendations?

Thanks in advance!
-Jeff

Heres a start if you happen to be using postfix.. but since you didn't specify all I can do is speculate..

http://www.postfix.org/ADDRESS_CLASS...y_domain_class

I don't think a backup mail server typiclaly delivers to mailboxes.. it holds everything in a queue to be forwarded to the final delivery destination when it comes back up.

Most mail clients don't have provisions for multiple servers for a single email address.. I suppose you could configure thunderbird for multiple mail accounts with a single storage location and it would check both servers and download mail from both servers.. the users wouldn't really be able to tell the difference..