TL;DR: My VPN's IP address has been blacklisted by Google, which has prevented Thunderbird from sending e-mails.
Adding a Gmail account to Thunderbird was easy enough and Thunderbird started syncing my messages with Google straight away. Sending a new message was a different story. Here are the Google SMTP server settings for a SSL/TLS connection:
When I clicked send on the e-mail, Thunderbird tried sending it with the following message:
Connecting to smtp.googlemail.com
You wait a few seconds and… timeout:
Sending of message failed.
The message could not be sent because the connection to SMTP server smtp.googlemail.com timed out. Try again or contact your network administrator.
The internet told me to change the connection timeout settings, verify my SMTP settings, unlock my Gmail account with the CAPTCHA or disable two-factor authentication (I am using App passwords). But none of that worked.
I found an article about IP address blacklisting and that finally rang a bell: my VPN!
I run almost all my internet traffic through a VPN and didn't think it was related to my Thunderbird Gmail issues but, sure enough, I disabled the VPN and there it went, e-mail sent. Guess spammers like my VPN provider as well.
Now, I'm not sure why this was never an issue with the Mail app - maybe Apple uses different SMTP settings when you configure a Gmail account?
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