Google Mail requires OAUTH2 for authentication. It requires several steps to configure.

You will start at the API credentials manager:
https://console.developers.google.com/projectselector/apis/credentials

You first need to make a project. My example calls this CrushFTP-Test.



Next select create credentials, and choose the Web Application type.



It will warn you if you don't have an "OAuth Consent" screen configured, so go there and configure that screen too.



When configuring the credential, you have to tell Google the domain you will be originating from when creating the auth token, so this is the URL you use for server administration. Just the protocol://dns_or_ip:port Don't have a trailing slash or it will complain.
You also need to put in the redirect URL of where google is going to send you back to after approval. This needs to follow the syntax in the example. Specifically your domain needs to end with:

?command=register_google_mail_api