Mac OS X Mail.app doesn’t properly work out of the box when connecting to a Courier IMAP (Plesk) mail server. If Mailbox Behaviors is configured to save Drafts, Sent, Junk, or Trash on the server then Mail.app will constantly report the error The message could not be saved. Quite annoying and, in the case of the Sent mailbox, Mail.app will happily just send the message to the bit bucket and not tell you.
To solve the problem go to Preferences, Advanced, and use INBOX as your IMAP Path Prefix. Problem solved!
Update (03/29/2007): Please note that this fix is not a magic bullet and is specifically for Courier IMAP mail servers running on Linux managed by a Plesk control panel environment because that’s the only place I’ve tested it. There are literally hundreds of different IMAP servers out there and each one has its own quirks and configuration options.