Hello there

I'm looking for some solution about how to make 2 Mail servers (or more) to act as a one, in case when one of them goes offline, that other continue to work.

Everytime when the new Mail comes, it should be copied on the both servers.
That should be easy.
But what happens in a case, when one Server goes down, and users log themselfs to another one, and delete some messages (readed etc.), that changes wouldn't be applied to the server that is offline.
That means when the server comes up, it will be with the same email-list content it was before going offline/down/freeze.

What is the best solution to keep mail Server always online thru other server in chain, without loosing Mails.

I'm using: qmail & courier-imap

Thanks in advance

For incoming mails that won't be a problem. The nature of MX records means you can give each server their own MX record - one primary, one secondary - if the primary one fails, other mail servers will send to the other one. However, you're talking about clustering, so to keep both servers synchronised, you'd need some form of shared storage - whether an NFS share mounted on a machine common between the 2, or a NAS device, or whatever.

Of course then your shared storage becomes a single point-of-failure, so you'd then need 2 of those!

The other option is to keep one server sitting as a "hot-spare" and continually rsync the user's home directories off the primary. Use a low TTL value on your mail server's DNS records. If the primary server fails, your hot-spare will be more-or-less up-to-date, so just switch the DNS over, and within a few seconds your clients will switch over. When you're primary one is backup and running, keep that one as the hot-spare instead, or rsync the directories back again when you get a lull in client traffic.