What a 4xx means
SMTP draws a strict line: 5xx is final, delivery has failed. 4xx means “not right now” — the sending server keeps the message queued and retries over hours. Only when that fails for days does a final bounce appear.
A single 421 is therefore no cause for concern. When they pile up or persist against the same receiver, a judgement is behind it.
The usual triggers
- A volume jump. Going from 50 messages a day to 5,000 looks like a compromise to any large receiver. New or long-dormant IPs must be warmed up slowly — over weeks, not days.
- Complaint rate. Recipients clicking “spam” weigh heavily. Google reacts to fractions of a percent.
- Missing authentication. Without consistent SPF, DKIM and DMARC the trust level is low and the throttling threshold is reached correspondingly early.
- Too many parallel connections. Some systems open dozens of simultaneous connections to the same destination. Large receivers cap that hard.
- Undeliverable addresses. A high hard-bounce rate reads as a poorly maintained list.
What not to do
Shortening retry intervals or raising the number of parallel connections tightens the throttling — that behaviour is exactly what distinguishes a normal mail server from a bot. Postfix's defaults are deliberately restrained here and should stay that way.
Sensible steps
Check the fundamentals first: is the IP on a list? Is reverse DNS set and confirmed? Do SPF, DKIM and DMARC pass consistently? Those three are the levers actually in your hands.
With Google and Microsoft it also pays to register with their respective postmaster tools — they show complaint rate and reputation scoring that stay invisible from the outside otherwise.