Gmail is rate limiting Cloudflare
upstream (gmail-smtp-in.l.google.com.) temporary error: Unknown error: transient error (421): 4.7.28 Gmail has detected an unusual rate of unsolicited mail. To protect 4.7.28 our users from spam, mail has been temporarily rate limited. For 4.7.28 more information, go to 4.7.28 https://support.google.com/mail/?p=UnsolicitedRateLimitError to 4.7.28 review our Bulk Email Senders Guidelines.
Email sender guidelines - Google Workspace Admin Help
The guidelines in this article can help you successfully send and deliver email to personal Gmail accounts. Starting in 2024, email senders must meet the requirements described here to send email to G
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And looking at other email topics in here... Most major providers are rate limiting CF. Making the functionality practically useless.