421 4.7.0 [TS03] — Temporarily deferred due to volume or complaints (Yahoo)
Retrying later can work — at a lower rate, not the same one.
The reply
421 4.7.0 [TS03] Messages from <IP> temporarily deferred due to unexpected volume or user complaints.A warning shot. Yahoo is deferring rather than rejecting, which means this can still be recovered — but it names the two reasons directly, and ignoring it is how senders arrive at TSS09.
What it means
A 4xx deferral, so the message is not lost and retrying is legitimate. What makes this one worth reacting to rather than simply retrying is the wording: unexpected volume or user complaints. Both are your side. A sender that answers by retrying at the same rate is answering the wrong half of the message.
Why it happens
- Volume from this IP rose faster than Yahoo expected from its history.
- Complaint rate climbing towards or past 0.3%.
- A large campaign sent in a short burst rather than spread out.
- Poor engagement on recent sends, which lowers the volume Yahoo will accept.
- A warming IP being pushed harder than its history supports.
What to do
- Slow down immediately — this is the moment where backing off still works.
- Retry with exponential backoff rather than on a fixed timer.
- Check your complaint rate through Yahoo's feedback loop and suppress the complainers.
- Spread large sends over hours and keep the per-hour rate steady rather than bursty.
- Remove unengaged recipients before the next campaign; they are what drives both signals.
Questions
Can I just retry?
You can, and it may deliver. But the reply names volume and complaints as the cause, and a sender who does not act on that usually meets TSS09 next — which does not clear by retrying at all.