This is apparently an unusual thing, I've searched for months and found no mention. Most contacts in my address books receive, forward, reply to emails in Thunderbird as you'd expect. There is one frequent contact, and possibly only one contact, person in my address books with whom this strange thing happens. And it happens not all the time, but frequently. A message will arrive with the header showing from 'Bob' and to 'Me'. As you'd expect. When I hit 'reply', the blank response message is created from 'Bob' to 'Me'. Opposite of what it should. Naturally, I assume Thunderbird is doing what it always does and I don't notice that, going straight to the reply. Until my reply hits my inbox seconds later, as being from 'Bob' to 'Me' and is not saved in the Sent folder. Usually I just forward that received mail to 'Bob', who at first got confused that I'm forwarding my own response. It seems to happen in batches, but then another message from 'Bob' to 'Me' will reply normally from 'Me' to 'Bob'. Always the same accounts. Both accounts are gmail, if that matters.