access your account settings. (Right click the account in the folder pane and select settings)

At the bottom of the accounts list is "outgoing server ( SMTP). Select the entry and then the outgoing SMTP account for GMail. Ensure the authentication method is set to oauth. (you will need to know your password to reset your user name and password via oAuth when you next try and send mail.

GMail is forcing oAuth these days for some users. It is apparently a staged migration. So perhaps that will help you.

Thanks for the reply, Matt! oAuth2 is already set as my authentication method. Today I'm able to send email from my gmail web page...but still can't in Tbird....but I can receive emails with no problem. I've downloaded a new copy of Tbird to my laptop (it's only been installed on my office desktop) to compare server settings with it and the copy that's been working just fine on my desktop since I bought a new tower 18 months ago. Something had to have changed...and from the number of people reporting the same problem, not just in my computer. I have a feeling the most recent update is the source of our woes.

The next logical step is to log the SMTP session. https://wiki.mozilla.org/MailNews:Logging that log will record all the "conversation and commands except user name and password bits.

However I would suggest you check in google at https://myaccount.google.com/security check recent security events, access for third party apps etc. It is possible Google are denying access for some reason. Perhaps you are using a VPN and they don't think the device is you for instance.

Unfortunately Google are prone to making changes to authentication that we are not aware of until after the event. But the last of those I am aware of was in 60.8

I fixed it! My authentication for both incoming and outgoing mail had always been "normal password" ...apparently changed to oAuth2 only for outgoing in the most recent update. When I saw that "normal password" is the authentication method for incoming and is still an option in the SMTP settings, I decided I had nothing to lose by switching it back....and IT WORKED!!! I can send emails again! And I'd bet the same "fix" will work for everyone else having this problem.

Thanks again for jumping in...without your pointing me to the authentication method, it would never have occurred to me to look there.

--Peggie

Google has already announced they will actively refuse connections requiring the less secure apps setting to be used on new gsuite accounts from June. https://gsuiteupdates.googleblog.com/2019/12/less-secure-apps-oauth-google-username-password-incorrect.html

That will extend to all gsuite beginning in 2021. While I have seen nothing concrete, I would expect non paying customers (gmail) to be cut off at the same time as the paying customers are (gsuite) So using Normal password is not a long term solution.

Google already default the less secure apps setting to off every time they do one of those security checkups, then the email just stops.

The answer is to use oAuth for everything and work out what the problem is, not revert to normal passwords until one day your mail just ceases to function permanently.

It may not be a longterm solution, but thanks to your input I'll know what to do when normal password ceases to work. And I'm very grateful to you for ALL this information!

--Peggie