Mail forwarded to gmail is rejected

Hi, gmail started rejecting mails forwarded by Cloudflare using the following error:

SPF status
pass
DMARC status
pass
DKIM status
pass

Rejected reason:

Unknown error: transient error (421): [2405:8100:c000:1::1b:df 15] Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily rate limited. Please visit https://support.google.com/mail/?p=UnsolicitedRateLimitError toreview our Bulk Email Senders Guidelines. f12-20020a5b01cc000000b009f31283d027si6921418ybp.257 - gsmtp

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Hi @nikosft thanks for your question, I have forwarded this concern to our engineering team and will let you know what we hear.

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Hi @nikosft, our engineering team say that the error is transient/temporary and that the email will be automatically retried. Let us know if you have any further concerns.

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Hi @erisa-cf , how transient should the issue be? I’ve got a few replies on one thread that has been retrying for ~3 days now, I would have expected it to go through by now.

An interesting data point: all the failed emails were sent from gmail addresses, even though there were other email providers on the thread. That being said, not all the gmail-sent replies were blocked

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