Continuation of my prior post: https://support.mozilla.org/en-US/questions/1516342 Thunderbird has a routine problem with being unable to log into various mail servers. I have four accounts and they all seem to share this problem. Most recent update: I've tested this in Windows Safe Mode, and the problem has not appeared. Since I can't do any useful work in Safe Mode, it's hard to give it a thorough test, but so far so good. Also, I have removed AVG antivirus because it was redundant. I have seen no improvement since it was removed. Can this problem be resolved?