Just a guess: Try smtp.gmail.com. I've had no trouble with gmail and Oauth2.

I am trying to set up a fresh install of Thunderbird on my Windows 10 computer with smtp.google.com as my outgoing server but OAuth2 is not showing up as an authentication method option.

And why do you think smtp.google.com is the correct server name? Did you enter that manually? If so, why? Let the New Account Wizard do it's thing, it will automatically discover the correct settings, incl. OAuth2 authentication. And you'll find that the correct outgoing server for Gmail is smtp.gmail.com.

I changed my SMTP server to smtp.gmail.com, restarted Thunderbird and OAuth2 came up as an authentication option. Chose that, restarted Thunderbird, and it appears that I can now send mail.

christ1 - smtp.google.com was the way it was configured on my old PC that was working. That had OAuth2 as an option. I have 20+ years of Mozilla/Thunderbird emails - when I imported that account from my old PC it didn't import the account settings so I had to do it manually.

christ1 - smtp.google.com was the way it was configured on my old PC that was working. That had OAuth2 as an option. I have 20+ years of Mozilla/Thunderbird emails - when I imported that account from my old PC it didn't import the account settings so I had to do it manually.

Just because it worked does not make it the correct setting. There are lot of folk out there still using googlemail server names as well from gmail beta days. They also have issues with changing to oauth because which servers it works with and thus is offered for is hard coded into Thunderbird. There is nothing dynamic about it.