I have this as well just upgraded to 1.77, thought it was related to that. I think it may actually have to do with new round of windows updates and virus filter?
It sounds like some sort of mail looping issue. If you are running plesk it might be the DrWeb issue (to which they have a patch). Perhaps inspect the logs to determine where the messages are coming from.
I use SpamAssassign Outlook executable to filter spam, wondering if that is the problem. I've disabled all filters. Will let you know if it comes back. AV was disabled and it froze again.