Strange "undo delete" issue

That is not a known issue.
Is it reproducible?
What is the mailbox location of the message when you delete it?

It disappears. Not on any folder anywhere. Says it's undeleted but it doesn't pop up in "inbox" and doesn't show up in "Deleted mail" or trash.

I'll take a series of screen shots and show you.

1. Test message formulated
2. Arrives in inbox- deleted
3. Undelete clicked
4. Nothing in inbox
5. Nothing in "deleted mail"

This really is strange. Try this: Open a new window using ⌥⌘N and see if it shows the same behavior.

Have you tried restarting MailMate?

Here is a low level way to verify whether or not MailMate has the messages on disk. Go to the following folder:

/Users/<login name>/Library/Application Support/MailMate/Messages/

Going further down the folder tree you should be able to see the messages in the INBOX. (Do not delete or move any of these messages.)

will try.