An SMTP 421 response means the receiving server is refusing the connection right now, not refusing your message specifically. RFC 5321 defines 421 as "Service not available, closing transmission channel," issued when the server has to shut down the session, whether that is scheduled maintenance, a load limit, or the server deciding your IP is sending too fast.
421 is a base reply code, not an enhanced status code on its own, and it is almost always paired with an enhanced code such as 4.7.28 that narrows down why. RFC 3463 classes any 4yz reply as a persistent transient failure: the message as sent is valid, held up by a condition expected to clear.
Unrelated to your sending behavior specifically. It tends to affect delivery to that receiving domain broadly for a window, not just your IP, and clears on its own once the condition passes.
Specific to your sending pattern. Gmail issues a 421 4.7.28, for example, when it decides an IP or domain is sending at a rate its current reputation does not support, a different situation from domain-wide capacity even though the base code looks the same.
Unlike a permanent 5yz code, a 421 is legitimately worth retrying once, with backoff. But if the real cause is a reputation throttle and you retry aggressively across the same recipients at the same domain, that can compound the exact sending pattern that triggered the throttle in the first place. The ramp rate I actually use is gated on engagement holding at each step rather than a fixed retry count, for the same reason.
No. Blocking is usually a 5yz permanent refusal. A 421 is the server saying not now, and RFC 3463 classes any 4yz reply as a persistent transient failure rather than a permanent one.
Enough for a genuine capacity blip to clear, with backoff between attempts. If the same domain keeps returning 421 across multiple sends over hours or days, that is no longer a transient blip; it is a rate signal worth reading rather than retrying through.
The 421 itself is the provider protecting its own queue, not a reputation penalty by itself. Repeated 421s against one domain are frequently a symptom of a reputation problem already forming, worth checking rather than assumed to be the cause on their own.