I have had this problem for a long time. Currently using Thunderbird version 68.8.0 While mailmerging - and I merge to the outbox and then send the messages when convenient, I have approx 154 messages in the outbox. After approximately 80 or so messages sent, I get the message that password is invalid and I enter the password several times, normally no joy. I then close the program, wait for a couple of minutes, start the program and then after a couple of tries of entering the password, the mail merge continues. Very frustrating, and not sure why this is happening - the password is always correct (I check with my gmail account and it works perfectly). I use the mail merge frequently so this is a hassle. Anyone have any ideas?