/
421 4.7.28 Gmail has detected an unusual rate of email.
Gmail rate-limited the message. The first 421 4.7.28 row on the SMTP error list is an unusual rate of email.
- Other
421 4.7.28rows name an IP address, an IP netblock, a DKIM domain, an SPF domain, a URL domain, or a repeated Message-ID.
The title is the first 421 4.7.28 row on Google's SMTP error list, last updated 18 September 2026 UTC. The other rows on that same status are not this sentence. The 550 5.7.28 page is the blocked twin that names an IP address.