Ok, thanks. I had completely forgotten about that other thread. I guess I don't really understand exactly what a heuristic detection is. The message referenced in the other thread from a month ago was flagged because of a suspicious subject line. This time there was no reference to a subject. It seemed to indicate that the message had a virus with it.
I guess I now finally understand that there is no option to quarantine email that gets tagged by Avast! Sorry! There may be valid reasons for this, but from a user standpoint, it would be a good option so we could at least have access to it if it was a false positive. For example, the message from a month ago turned out to be valid and harmless. I was able to contact the person who sent it and they just resent it without a blank subject and it came through. However, I sometimes get messages without subjects just fine. The other weird thing is that even when choosing the delete response to the Avast popup, it pops up again and I have to delete it twice before email receiving continues. This is similar to if I choose "continue" which just gives me an endless loop until I choose "delete".