Problem Statement
We are using SMTP Gmail service as our custom email provider. We did not receive emails. The log shows the error:
Invalid login: 535-5.7.8 Username and Password not accepted.
Solution
You need to provide a Gmail App Password for some apps to allow them to connect to Gmail service since they ask for it if it is a non-google-app or a device trying to log in.
Gmail documentation guide: Sign in using app passwords - Gmail Help