Yep I was shocked that SendGrid doesnt do this I don't even expect them to keep a list of IDs forever, just some best effort like "we don't send anything with

Yep I was shocked that SendGrid doesnt do this I don't even expect them to keep a list of IDs forever, just some best effort like "we don't send anything with the same id twice in a one-hour window" Gmail's API did support this I believe, and then ofc my org transitioned to Microsoft so my little homemade email service quit working