I’m hoping someone from Gmail can reply off list. We (Stanford University—stanford.edu) are receiving a large number of deferrals when trying to delivery to Gmail.com. The smtp code returned is:
421-4.7.28 Gmail has detected an unusual rate of mail originating from your SPF The best educated guess I can make as to that error is that when our Office365 environment last weekend began doing SRS and rewriting the sender address on forwarded messages (many users forward their M365 mailbox to a personal Gmail address) this was as a spike in mail from our domain, although the actual volume of mail should not have changed. In any case, the SRS was turned back off yesterday but we are still effectively rate limited, with 20-30k messages pending delivery to gmail.com. The response from the gmail.com help site was that if any mitigation was warranted it would take two weeks and that there was no way to reply to that “ticket.” Thanks, Jason Cowart University IT Stanford University
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