Trend used to do that to mail in Thunderbird. What antivirus are you using?

Hello Matt & thank you. I've been using AVG for years now. Never had this problem until a few days ago. I have three email accounts that I use with Mozilla. Only one of the accounts has this situation. Your reply came to my nullnvoid account; & it went directly into the trash folder!

In that case, try disabling AVG (or some setting in AVG).

Thanks Wayne, for your reply. It's strange that there's only one of the email accounts that's affected. I'm not sure if disabling or changing a setting in AVG would help. But I'll try it anyway, to see what happens. I even thought of deleting the affected account, and later re-installing it, to see if it would make any difference.