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Error codes

550 5.7.28 There is an unusual rate of unsolicited email originating from your IP address.

Gmail blocked the message. It says the sending IP produced an unusual rate of unsolicited email.

The title is the 550 5.7.28 row on Google's SMTP error list, last updated 18 September 2026 UTC. 421 4.7.28 is the rate-limited family. This row is a block, not a 421.