421: IP blocked for mail to unknown addresses

What it means

IONOS watches the delivery rate of each sending server. When a large share of its mail goes to addresses that do not exist at IONOS, it blocks the IP from sending to IONOS customers and answers with 421, a temporary refusal. IONOS adds a second reason for the same reply: your sender address has been reported as a spam sender in the past. Either way the cure is the same. Clean the list of inactive and outdated addresses, then restart sending later.

Common causes

• Your list holds many addresses that no longer exist or never did.
• Hard bounces from earlier sends were never removed.
• Your sender address was reported as a spam sender in the past.

How to fix it

1. 01Remove inactive and outdated addresses from the list, starting with every hard bounce.
2. 02Pause sending to IONOS-hosted domains while you clean the list.
3. 03Restart sending later, at a lower rate at first.
4. 04Collect new addresses with confirmation (double opt-in) so the problem does not return.

Sources

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

• SMTP error messages of the IONOS mail servers (IONOS Help)ionos.com

Further reading

• Email list hygiene