421 4.7.0 Temporary Failure

Service not available / connection rejected (policy)

The receiving server is temporarily refusing the connection, often due to rate limiting or a temporary reputation-based policy restriction rather than anything about the specific recipient.

Common causes

Sending IP has exceeded the receiving server's connection rate limit.
Receiving server is temporarily overloaded.
Sending IP has a borderline reputation triggering temporary throttling.

What to do

Retry after a delay with backoff. If this happens consistently from the same sending IP across many domains, investigate sender reputation rather than treating it as a per-recipient issue.

FAQ

Is a 421 the recipient's fault or the sender's?

Almost always the sender/sending-IP's situation (rate or reputation), not anything about the specific recipient address — it occurs before the server even evaluates RCPT TO.

Other temporary failure codes

450 4.2.0

Mailbox temporarily unavailable

451 4.3.0

Local error in processing / greylisting

452 4.2.2

Mailbox full (over quota, temporary)

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