I've been toying with an interesting idea at work, and since I couldn't come up with any ideas on my own (or my circle of friends/coworkers), I thought I'd present it here for more opinions (since courier is our current choice of mail server).

Our goal is to set up one server at our colo for fast handling of incoming mail. This server would do spam/virus scanning/blocking to reduce the download load on our in-office DSL connection. A second on-site server would be set up to receive mail from the first server for faster access from machines on our LAN.

That much I think courier can handle (or so I've inferred from conversations on this list), and will most likely be what we do, but what I'm wondering is if anyone has heard of a way that the servers could be replicated (like a database does) so that people offsite could check their mail (IMAP) at the colo server instead of coming in via our slow DSL connection. So far, I've heard of several one-way options (eg. rsync or drbd), but nothing capable of passing changes both directions to keep both the colo and local servers completely in sync.

Though I doubt that this could actually be done, it does at least present an interesting concept idea, so I thought I'd put it up for discussion.

-Chris