How do I get around HttpError 403 Insufficient Permission? (gmail api, python)

Even though the accepted answer is 100% correct. I think it's worth pointing out that why that's the case.

When you authorize a gmail service client, you can specify several different scopes: All, compose, labels, etc...

These are all listed here: https://developers.google.com/gmail/api/auth/scopes

The scope mentioned in the answer provides complete gmail access.

Solved it by changing the SCOPES line to:

SCOPES = 'https://mail.google.com/'

Email sending works perfectly