421: temporary reject under an exceptional blocklist

What it means

GMX keeps some blocklists for exceptional situations, when it sees misuse it wants to stop quickly to protect its infrastructure. Cases r0001 and r0003 are the temporary form: the reply starts with 421, so your server keeps the message and tries again later. The case page gives no specific trigger, only the instruction to stop whatever is sending unwanted connections or spam from your system. Retries will keep meeting the same answer while the block is in place. After the cause is dealt with, GMX asks you to get in touch through the form on the case page.

Common causes

• Your server or network sent spam or other unwanted connections to GMX.
• A compromised account, script or open relay on your system is being misused.
• Your IP address sits in a range GMX has blocked to stop an ongoing abuse.

How to fix it

1. 01Check your outbound queue and logs for mail you did not intend to send, and stop it.
2. 02Close any open relay, reset compromised passwords and remove scripts that send without authorisation.
3. 03Let your server retry on its normal schedule rather than resending by hand.
4. 04Once the cause is fixed, use the contact form on the case page (or postmaster.gmx.net/en/contact) and include the full last line of the error, your IP addresses and your domain.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• GMX Postmaster case r0001: 421 Service unavailable, Administrative rejectpostmaster.gmx.net
• GMX Postmaster case r0003: 421 Reject due to policy restrictionspostmaster.gmx.net

Further reading

• Blacklist checker