I just imported some years of a mailing list to an IMAP mailbox using MailMate. The source data was from the pipermail web UI which lets you download 'text files' that are near enough to .mbox format to import, but don't have all the headers. (http://rubyforge.org/pipermail/rspec-users/)
I tend to sort my mail by 'Date Received'. After the import, I noticed Mail.app (annoyingly) shows all 30,000 messages as being received "Today". MailMate, on the other hand, (awesomely) shows them all as being received on the actual date of the message (the date that the message's Date header field indicates). Viewing the message source, I see that only a minimal set of headers is actually present, and there are no "Received:" headers at all. Am I correct in thinking that, if no "Received:" headers are actually present, MM uses the "Date:" field as the date received (for the purpose of sorting). And probably Mail.app decides to use the first date it saw the message? In any case, I am pleased by this aspect of MailMate's behavior, it is something that is often problematic with IMAP clients when doing stuff like this.
