Transient (4xx)
421 4.7.0
Temporary policy rejection
A generic temporary policy-based rejection, often related to sender reputation or rate limits.
Common causes
- Sending IP or domain reputation is under review at the receiving provider.
- Sending too fast for a cold or low-reputation IP (common right after starting a new sending domain).
What to do
- →Warm up new sending domains/IPs gradually rather than sending full volume immediately.
- →Check your domain isn't on a public blocklist and that SPF/DKIM/DMARC are all correctly configured.