Transient (4xx)
421 4.7.28
Rate limited due to reputation (Gmail-style)
A temporary block some large mailbox providers issue specifically for suspicious sending patterns or IP reputation, distinct from a generic overload response.
Common causes
- Sudden volume spike from a previously low-volume sender.
- Sending IP recently associated with spam elsewhere.
What to do
- →Slow down and warm up sending volume gradually rather than spiking. Confirm SPF/DKIM/DMARC are all correctly configured for the sending domain.