Not sure what you meant by local backups, but all messages retrieved by MailMate are stored in a plain directory as RFC 822-based EML file in ~/Library/Application Support/MailMate/Messages (which is relocatable from within the app). It also downloads all emails by default (but not the attachment).
Wait - you mean it is possible to properly archive messages on my mac now? Without having to run a local IMAP server? If I remember correctly that was the only solution at the time. Time to take another look it seems.
