SMTP Gmail API OAuth: Token has been expired or revoked.(invalid_grant)

[S7 Mailer]

Hi there, hope I'm in the right place.

We use OAuth to log in to a Gmail account on websites to send email via SMTP from forms. This was to get around PHPMail issues and to reduce spam. We've had this set up for years and just today we've started to get this error from all forms:

Token has been expired or revoked.(invalid_grant)

What is the reason behind this? We have about 50+ Client IDs on there, all used to work fine. It's like they have unhooked themselves.

Do we now need to verify every single domain? Is it because of a spam issue?

Pretty desperate to get this email service back up and running. Many thanks for your time, looking forward to some ideas.

Thanks

[Nicolas (Google Cloud Platform Support)]

Hello,

I understand that you are experiencing an issue with the Gmail API,  as this group is intended for general discussion regarding App Engine and not for specific technical issue’s troubleshooting I you would have better assistance from another channel. I would recommend taking a look at recommend that you post your question on StackOverflow as described in this article.

This will allow you to have access to a large community of enthusiasts and experts to share ideas with and get support from. Please make sure to include all relevant details and error messages which would help the community troubleshoot.