I have had occasional problems with RFC compliance.
Most notably commercial organisations that neglect to set the 8-bit MIME header when they send 8-bit HTML. Older versions of Courier rejected the email, saying why. Newer versions default to hiving it off to an attachment. This was annoying my wife :-/. Fortunately, you can tell it not to do that.
That's the only problem I've ever had with any of Courier.
In a previous job, we had our own email system and needed the MTA to inject email into the app. PostFix was a bitch to make this happen - it was as though it was deliberately difficult to do. Courier, OTOH, was a walk in the park to do it.
I found Courier back when I was using qmail and needed an IMAP server that understood Maildir. At the time, Courier-IMAP was the only choice. When I did a system upgrade, qmail was Just Too Difficult to setup again (why do I have to patch qmail and why do I have to recompile a system binary to enable relaying?). The system alternative was sendmail - and all my mail was in Maildir format. :-/ By then, the rest of Courier had been released. It was much easier to configure. I've been using it ever since!
Wade.
