I use Thunderbird 38.5.1 in Ubuntu 15.04 desktop environment and have done so for a long time. Yesterday i enabled the Lightning calendar add-in. Shortly after doing this I discovered that outbound mail was not being sent. The mail would sit with the 'Sending Message' pop-up for some time then I would get a timeout error from the relevant ISP. I checked with the ISP to see if they had problems on their side - their was not - and, in fact, I have a Win 10 machine that has the same Thunderbird config (but no Lightning) and it was sending mail quite happily. These are the actions I have since carried out: 1. Restarted Thunderbird with Add-ins disabled - no difference. 2. Saved ./Thunderbird directory from my home directory, deleted and reinstalled Thunderbird, restored ./Thunderbird directory - no change 3. Renamed ./Thunderbird to something else and the start Thunderbird to create a new set of mail files and account details etc - no change Currently I can send a new email without problem, but I can not send a reply or forward an email without getting this timeout problem. Next step I guess will be to delete it all and start all over again. I am reluctant to restore my Linux ./Thunderbird save to my windows machine in case it causes that to stop working. I am currently staring down the barrel of having some years of mail archives that i cant use because they will break my system. Certainly I will never touch Lightning again. I hope someone has an idea of how to prevent this problem from re-occuring.