SMTP 421 4.7.0: rate limited – causes & fix

The receiving server temporarily refuses more mail from your IP because volume, reputation or authentication look suspicious – messages stay in the queue and are retried later.

Common causes

• A sudden spike in volume, e.g. a newsletter from a new IP without reputation.
• Missing or mismatched PTR (reverse DNS) record; Gmail is especially strict for IPv6.
• Spam complaints, or a compromised mailbox sending spam.
• Missing SPF/DKIM authentication makes the rating worse.

How to check it

Check the queue size

mailq | tail -n 1

Find the senders with the most mails in the queue

mailq | awk '/^[0-9A-F]/ {print $7}' | sort | uniq -c | sort -rn | head

Check the PTR of the outgoing IP

dig -x 203.0.113.10 +short

What actually helps

1. Let Postfix handle retries – do not run postqueue -f over and over, it makes throttling worse.
2. Set a PTR that matches the HELO name and make that name resolve back to the IP (forward-confirmed rDNS).
3. Lock compromised accounts, reset passwords and remove spam with postsuper -d.

FAQ

Are messages lost on a 421?

No. 4xx is a temporary error. Postfix keeps the mails in the queue and retries until maximal_queue_lifetime (5 days by default).

Why does it only happen over IPv6?

Gmail requires a valid PTR and authentication for IPv6 senders. Without a PTR, either add one or make Postfix prefer IPv4 with smtp_address_preference = ipv4.

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Full page with every command and the German/Italian version: daniele-michelin.com/en/errors/smtp-421-4-7-0/

First published at daniele-michelin.com. The error lexicon covers HTTP, DNS, TLS, mail, database, PHP, nginx, systemd and Nextcloud, in German, English and Italian.