New G-Mail rules came into effect around 2022 that require further setup to use as mail relay.
https://askubuntu.com/a/1398451/395258
Go to the security options of the sending account: https://myaccount.google.com/security
Activate 2FA (if you haven't already) and then select App Passwords: https://myaccount.google.com/apppasswords
Create your custom service (you can choose any name) and it will generate a special password.
Enter that password in the sasl_passwd file