What the receiver is telling you

The receiver closed the connection before the message was accepted. It is a temporary code by definition — 4xx — and your server retries. What it means depends entirely on the enhanced code and text that follow: connection limits, rate limits, server restarts, greylisting. On its own, 421 tells you the receiver was not willing to talk right now.

How to handle a soft bounce

Your server or sending tool retries soft failures automatically, usually for one to three days. Most clear on their own. The mistake is leaving an address that soft-bounces on every step in an active sequence: each attempt is logged, and a mailbox that has been full or unreachable for two weeks is not going to read a follow-up. Three consecutive soft failures is the point to remove it. Track soft bounces separately from hard ones; they are a different signal and receivers weight them differently.

What to do

1. Read the enhanced code after it. 4.7.0 and 4.7.28 are rate limits and need volume cut; 4.3.2 and 4.4.2 are transient and need nothing.
2. If 421 persists from one provider for hours, treat it as a soft block.
3. If it is 'too many concurrent connections', your tool is opening more parallel connections than the receiver allows — reduce concurrency.

don't Do not treat a 421 as delivered or as bounced. It is neither until the retries resolve it.

Decode the whole bounce

Paste the complete failure notice — every code and the receiver's wording — and get each part explained together.

Common questions