This should be fixed in TB 68.2.1, or you can apply a workaround:

https://support.mozilla.org/en-US/questions/1271783#answer-1262722

Many thanks sfhowes, upgrading to 68.2.1 fixed it.

I am using TB Version 68.5.0 (64bit) Win 10, and getting the same error message, specifically "The message could not be sent because the connection to Outgoing server (SMTP) smtp.gmail.com was lost in the middle of the transaction. Try again." The problem occurs across several gmail addreses. All suggestions are appreciated.

Jim The Old Guy said

I am using TB Version 68.5.0 (64bit) Win 10, and getting the same error message, specifically "The message could not be sent because the connection to Outgoing server (SMTP) smtp.gmail.com was lost in the middle of the transaction. Try again." The problem occurs across several gmail addreses. All suggestions are appreciated.

Change the authentication to OAuth2 in Server Settings for the incoming server and also in Outgoing Server (SMTP), in Tools/Account Settings. Remove the passwords in Tools/Options/Security/Passwords/Saved Passwords, restart TB, and enter the regular account password in the OAuth2 browser window when prompted. Cookies must be enabled in TB Tools/Options/Privacy.

Thanks so much sfhowes! For other reasons, I use my local ISP to receive mail (I have my gmail mail forward to those accounts) so am reluctant to change my incoming pop settings to 0Auth2. I also am having no difficulty receiving email forwarded from my local ISP. Could I possibly perform all other changes you suggest without changing any of my incoming settings? ---Jim

You can't change to OAuth for your local ISP, but if you are sending through the gmail smtp, as the error indicates, you can set OAuth on smtp.gmail.com. The benefit is you don't have to enable less-secure apps in gmail.

Are you sending from a non-gmail account through the gmail smtp? Unless the account is added as a sender in gmail, gmail will switch out the account address for the gmail one.

Thanks sfhowes,

OK, my Outgoing Server settings are now:

Description: Google Mail Server Name: smtp.gmail.com Port 465 User Name: jam****ll47@gmail.com Authentication Method: 0Auth2 Connection Security: SSL/TLS

Yes, gmail changes my local address to the gmail one when I send (back when I was able to send) which is what I prefer.

Does this look ok?

Looks OK - let's hope it works.

Wow it worked!!!! You are my hero! Thanks so much sfhowes!

I recently started getting these messages also:

Message could not be sent....... Outgoing server (SMTP) example.net was lost in the middle of the transaction. Try Again.

This is not related to Gmail. It is on my own email account on my own (shared hosting) website. I do not use OAuth.

So what else could be causing these lost connections?

K4HM - Hank said

I recently started getting these messages also: Message could not be sent....... Outgoing server (SMTP) example.net was lost in the middle of the transaction. Try Again. This is not related to Gmail. It is on my own email account on my own (shared hosting) website. I do not use OAuth. So what else could be causing these lost connections?

Indicate the settings you have for the account, obscuring personal data, and the antivirus program.

http://kb.mozillazine.org/Cannot_send_mail

Using V 68.6.0 - I had to change my Gmail PW with a browser the other day, then updated Thunderbird which then would receive email but not send.

Beat my head on the wall for two hours trying to solve the mystery. Changed from 0Auth2 to "Normal Password" and now it will send/receive through TB mail client. Seems odd since it was working with 0Auth2 just fine until the PW change. I tried deleting the account in TB and starting fresh with the Gmail added as a new account, still no luck.

Oh, well. Guess I live with "Normal Password" authentication.