Completely Crashing

Sorry to hear that. Do you have a crash report which you can attach to this ticket? (I don't get the ones sent to Apple.)

Yes. pls see below

That doesn't look good. It appears MailMate is trying to delete a message from its database, but this fails. Apparently because it doesn't know the location of the message. This should never happen and therefore is an indication of some kind of database corruption.

When you write “spam filter app”, do you mean SpamSieve or something else?

I'm not sure yet, but I think the only way to get back on track is to force MailMate to rebuild its database by refetching all emails from the server(s). This can be done by moving/removing the following folders:

~/Library/Application Support/MailMate/Database.noindex
~/Library/Application Support/MailMate/Messages
~/Library/Application Support/MailMate/Messages.noindex
~/Library/Application Support/MailMate/Attachments

When you relaunch MailMate then it'll start fetching emails from the IMAP server(s).

application support folders for MailMate do not exist on my system. Any thoughts?

You are probably having issues finding the /Users/<user>/Library folder (~/Library). See more here.