About GMail SMTP OAuth2 Scopes

The GMail OAuth2 scopes relating to sending email are:

• "https://mail.google.com/": This scope grants full access to the user's Gmail account, including the ability to send emails, read emails, and manage labels. It is the most comprehensive scope for Gmail access.

If you only need to send emails and want to limit the scope to just that, one would think you can use the more restricted scope:

• "https://www.googleapis.com/auth/gmail.send": This scope allows your application to send emails but does not grant access to read or modify the user's emails or labels.

However, the"https://www.googleapis.com/auth/gmail.send" scope cannot be used in the SMTP protocol.

The issue is due to the way Gmail's OAuth2 scopes interact with the SMTP protocol. While the "https://www.googleapis.com/auth/gmail.send" scope is intended to allow sending emails, it does not grant sufficient permissions for authenticating with Gmail's SMTP server. Here's why:

Why "https://mail.google.com/" Works:

The "https://mail.google.com/" scope grants full access to the Gmail account, including the ability to authenticate with Gmail's SMTP server. This scope is broader and includes all the necessary permissions for SMTP authentication and email sending.

Why "https://www.googleapis.com/auth/gmail.send" Fails:

The "https://www.googleapis.com/auth/gmail.send" scope is more restrictive. While it allows sending emails via the Gmail API, it does not provide the necessary permissions for SMTP authentication. Gmail's SMTP server requires additional permissions beyond just sending emails, which are only granted by the broader "https://mail.google.com/" scope.

Key Differences:

1. SMTP Authentication Requirements:
   • Gmail's SMTP server requires full access to the account for authentication, which is only provided by the "https://mail.google.com/" scope.
   • The "gmail.send" scope is designed for use with the HTTP-based Gmail API, not the SMTP protocol.
2. Scope Limitations:
   • "gmail.send": Allows sending emails via the HTTP-based Gmail API but does not grant access to other Gmail features or SMTP authentication.
   • "mail.google.com": Grants full access to Gmail, including SMTP authentication and email sending.

Conclusion:

To use OAuth2 with Gmail's SMTP server, you must use the "https://mail.google.com/" scope. The "gmail.send" scope is insufficient for SMTP authentication because it does not grant the necessary permissions. If you want to adhere to the principle of least privilege, you can explore using the Gmail API directly instead of SMTP, as the "gmail.send" scope is sufficient for sending emails via the API.