450: deferred for mail to unknown addresses

What it means

Case r0901 gives the same reason as case r0902, with a 450 reply instead of a 421: your server wrote to too many non-existent addresses, so GMX is temporarily rejecting its mail. A 450 is temporary, and your server will retry, but the deferral will recur for as long as the list keeps producing unknown recipients. GMX treats a high share of unknown addresses as a sign of poor list practice or address guessing. The fix it gives is to remove hard bounced addresses from the list immediately and to follow its requirements for bulk senders.

Common causes

• Recipients that bounced as unknown are still on the list.
• The list contains typos or addresses that were never confirmed.
• The list is old, and many GMX mailboxes on it were closed or deactivated.

How to fix it

1. 01Delete every hard bounced address from the list right away.
2. 02Suppress addresses that have been inactive for a long time.
3. 03Use double opt-in for new signups.
4. 04Let the server retry the queued mail after the list is cleaned.

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 r0901: 450 Service unavailable, Reject due to policy violationspostmaster.gmx.net

Further reading

• Email list hygiene