Where you saw it: Mail logs showing deferred delivery, a verification result marked unknown, or a sending queue backing up against one destination domain.
Why it happens
- Rate limiting. The receiver has decided your IP is sending too fast and is slowing you down deliberately.
- The server is genuinely overloaded or in maintenance, which is what the code originally meant.
- Too many simultaneous connections from your IP, which many receivers cap at a low number.
- A reputation-based throttle, where a receiver limits senders it does not trust yet rather than rejecting them.
- Greylisting implemented with a 421 rather than the more common 450.
The fix, in order
Read which domain is returning it
A 421 concentrated on one receiving domain is a relationship problem with them. Spread across many is an infrastructure problem on your side.
Reduce concurrent connections to that destination
Most sending platforms let you cap connections per destination domain. Dropping from ten to two often clears the problem immediately.
Slow the send rate rather than the volume
Spreading the same number of messages over a longer window is usually enough. The limit is normally about rate rather than total.
Check your reputation with that receiver
Microsoft's SNDS and Google's Postmaster Tools both show whether you are being throttled for reputation. A reputation throttle does not respond to slower sending alone.
Retry properly, with backoff
A 421 is temporary and should be retried on an increasing schedule. Retrying immediately at the same rate is what triggered it.
How to know it worked
The short version
- Deferred messages to that domain clear on retry.
- The 421 rate drops after the connection limit change.
- Queue depth against that destination returns to normal.
Questions people ask
Does a 421 mean the address is bad?
No. It carries no information about the recipient at all. We report these as unknown and do not bill them, because a rate limit is a fact about us rather than about the address.
How should retries be scheduled?
With exponential backoff over several hours. Retrying immediately makes the throttle worse, and most legitimate 421s clear within a few hours.
Why does one provider throttle me and others do not?
Because each sets its own limits based on your history with them. A new IP gets throttled hardest by receivers that have never seen it before.