Please post a screenshot of your SMTP server settings. You can find that at 'account settings' and scroll down left side to 'outgoing servers (SMTP) . Thank you.

here. thanks!

Puzzling, that looks proper. Do you have more than one gmail account? If so, you may be crossing the two SMTP server links.

I have only one. The same error is with the Startmail account, despite that settings are also correct

Okay, this is over my head, having the same problem with two accounts indicates something else.

Do you have a VPN enabled ? If yes, then switch it off because the server will be expecting your computer IP address and if it's getting other random IP address then it may think you are not you - server may suspect you are a hacker and just block access.

If you really want to use a VPN then you need to create a tunnel so that Thunderbird can use it to bypass VPN.