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Hi @lyeffeth,
I hope you are doing well.
Please reconfigure the Post SMTP plugin with Gmail API, test it, and let me know.
Thanks and regards,
Support Team – WPExperts
We worked it out. I am not sure what you mean by reconfigure but when we got the error we checked the URLs in the Google API and the redirect URL was slightly different this time so we added it to the project and that worked. Not sure why it changed, but in case anyone else has the same issue.