This appears to be a known upstream bug (https://bugzilla.mozilla.org/show_bug.cgi?id=1371309), which was fixed in thunderbird 78 indeed. Let's keep this bug open until 78 is backported to bionic.
** Bug watch added: Mozilla Bugzilla #1371309 https://bugzilla.mozilla.org/show_bug.cgi?id=1371309 ** Also affects: thunderbird via https://bugzilla.mozilla.org/show_bug.cgi?id=1371309 Importance: Unknown Status: Unknown -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1899485 Title: cannot send message To manage notifications about this bug go to: https://bugs.launchpad.net/thunderbird/+bug/1899485/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs