Gordon Messmer writes:

Courier's strengths are: integration of most popular mail services: SMTP, IMAP, POP3, and webmail. Only have to deal with the configuration issues once, and you get the whole package working.

Also -- a firewall situation, where you want to be able to defensively screen incoming mail. Courier has a lot of code to deal with denial of service attacks. Although, by default VRFY and EXPN is turned on, anyone attempting to do a dictionary attack against a Courier server is in for a surprise. There are also several options for installing arbitrary aggressive mail filters.

Here's an experiment: enable one of the sample mail filter that gets installed by default, "dupfilter". Then try to mailbomb any account on the Courier server, using any lame ass mailbomb proggie. That's just an example. Write your own filters to deal with your own "unique" situations.

Or, enable the "perlfilter" filter, and try to send Courier a message with a long Date: header (the most recent Lookout Express exploit).

Two examples of what can be done here.

You don't want to use Courier sometimes when speed is an absolute requirement. Depending on the kind of mail traffic, Courier can be a lot slower than Qmail pushing a large load of mail out. But, in other situations Courier will spank the monkey out of Qmail. It all depends upon your kind of mail traffic, but don't take it for granted that you'll be able to push a load of mail through, in no time at all.

I can't think of a situation where I'd recommend sendmail over Courier :-)

I don't know much about Postfix, but from what I've heard, it sounds like it's slow, and needs lots of resources.