421: service not available
By Jose Pollman · Published · Updated
421 means the receiving server is not accepting your mail right now, and it is temporary: your server retries. It covers both genuine unavailability, such as overload or maintenance, and deliberate throttling of a sender it is unsure about. If it happens to everyone, it is an outage; if only to you, consistently, you are being rate limited.
The error
421 4.7.0 Try again later, closing connection
421 Service not available, closing transmission channelWhat it means
The receiving server is not accepting mail right now — overloaded, in maintenance, or deliberately throttling you.
Nothing is wrong at your end. The recipient or their mail provider has to act, and there is usually nothing you can do but stop sending.
Why it happens
Receivers use 421 both for genuine unavailability and as a polite way of throttling a sender they are unsure about. The two look identical, and the difference is whether it happens to everyone or only to you.
How to fix it
- Let the queue retry.
- If it is consistent and only for one destination, you are probably being rate limited rather than hitting an outage.
Related guides
- 5.2.2: the mailbox is full — The recipient is over quota. The 5.x.x prefix says permanent, but a full mailbox is usually temporary — treat it as soft until it repeats.
- TLS and DANE validation bounces — Microsoft would not deliver because the destination’s TLS setup failed validation — no STARTTLS, an expired certificate, a hostname mismatch, or a failing DANE or DNSSEC record.
- 4.7.0: temporarily refused on policy grounds — The receiver is temporarily refusing your mail for policy reasons — most often rate limiting, greylisting, or a TLS requirement that was not met.