Q: What does 'Gmail error 400: Mail service not enabled' mean?
A: This means the Gmail API that Speaking Email uses to communicate with Gmail is not able to be used for your account.
Most likely your email address is not a Gmail account. It is possible to be a Google Account but not a Gmail account (for example if you use the account for YouTube but have never enabled Gmail).
Otherwise, if you have G Suite (aka Google Apps for Work) your organisation administrator may need to enable the Gmail feature or Gmail API access.
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