If I compose a long e-mail or if Thunderbird is open for more than a short period of time it will not send an e-mail. I have a workaround. I have to close the e-mail and save it as a draft. I then close Thunderbird and immediately reopen, reopen the saved draft, and send it. That works only if Thunderbird is not open for more than a few minutes. It is like Thunderbird is disconnecting from the internet after a few minutes. I have been using this program for Years and have not changed any setting that I know of. This seems to have happened after an update several months ago.