Working OK here - although I did notice it doing some maintenance the first time I ran it after updating.Mail is not working properly for me in Beta5. It appears to be working, and does not crash, but I noticed that at some point after while new mail that I am sending or email that I am replying to do not actually get sent and do not show up in sent mail. When I try to close mail, it does not close and eventually requires a force quit. Once I force quit, then it will then start to load new messages that have come in since the bug started happening. I then need to recompose and send the new emails in question.
Caveat as usual for backup in case of data loss but trying rebuilding your Envelope Index.