Thunderbird version 78 and ESET Internet Security

If you use the Mozilla Thunderbird email program, and it won’t retrieve mail after updating to the latest Thunderbird version 78, this can be caused by a bug in the “ESET Internet Security” (or “Smart Security”) firewall/antivirus program.

In particular, if you use ESET and Thunderbird just hangs with a message saying “checking mail server capabilities”, ESET is almost certainly the cause. There are several threads on their forums about it ([1], [2]), and it’s not related to our servers — Thunderbird 78 works fine with our servers in general.

If this problem is happening to you, ESET has a page explaining how to workaround the bug and get Thunderbird working again.