“server has disconnected The server may have gone down or there may be a network problem” is shown – but the mail is send anyway.

strange enough: the error message is misleading – this is not a server problem – it is a thunderbird problem?

tested on: User-Agent: Mozilla/5.0 (X11; Linux i686; rv:52.0)
Gecko/20100101 Thunderbird/52.8.0
Static hostname: DebianLaptop
Icon name: computer-laptop
Chassis: laptop
Operating System: Debian GNU/Linux 8 (jessie)
Kernel: Linux 4.9.65
Architecture: x86

you can try this: reduce “Maximum number of server connections to cache” to 2, and work your way up until the error starts showing up, then reduce by 1 again to save levels.

creditz: https://linuxuserhelp.wordpress.com/2013/06/11/thunderbird-imap-and-server-xxxxxxxx-has-disconnected-the-server-may-have-gone-down-or-there-may-be-a-network-problem/