SMTP will no longer work with legacy passwords (basic authentication).

Starting in autumn of 2024, you and your users must use OAuth with third-party apps to access Gmail, Google Calendar, and Google Contacts. OAuth is a more secure access method. You will no longer use a password for access (with the exception of app passwords). Google is turning off access to less secure apps—non-Google apps that can access Google Accounts with a username and password (basic authentication). Using basic authentication makes accounts more vulnerable to hijacking attempts.