Yes, it's an interesting all-in-one mail handler which replaces sendmail/postfix, fetchmail, and pop3 server daemons. It has it's own
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Not my kind of strong Brownian motion producer. I happen to believe in seperate apps all doing their own thing, and within their own (security) context, very well. You only have to look at Exchange to see what can go wrong when a MTA is mated with an agenda, MDA, contact database, authentication database, spell checker, game server, coffee machine and $YOUR_DEITY known what else.
I agree with you up to a point. I think in general it's a bad idea to integrate many different uses into one package. I think Mozilla is another example of that and Firefox was the solution. In this case, however, I see "mail server" as a single use. I think the reason it works well here is that the daemons take a few tuning parameters but otherwise run unobtrusively in the background (as you would expect). All the interaction between the daemons and the rest of the world is handled through "filters" (think modules or plugins) which you have complete control over. I've been on their mailing list for 3 or 4 years and I see requests for new or additional features but I've never seen anyone ask "Can I replace the XMail pop3 server with $POP_SERVER? For all I know you may be able to swap out components the way you prefer, but nobody's ever asked. Jeff