554 5.1.1 — Mailbox not found (Yahoo)
PermanentRecipient's side
Retrying will not help. The message will not be delivered.
The reply
554 5.1.1 Requested mail action aborted, mailbox not found.No mailbox at that address. Permanent, nothing to fix on your server — but at Yahoo the rate of these matters more than at most receivers, because unknown recipients feed the same reputation that produces TSS09.
What it means
The address does not exist, was closed, or was deactivated. Yahoo does recycle some long-abandoned addresses, which makes an old list actively dangerous here: an address that once belonged to a subscriber can become a spam trap, and a trap hit costs far more than a bounce.
Why it happens
- A typo, or an address that was never confirmed.
- An account closed by its owner.
- An account deactivated after long disuse.
- An old list entry whose owner abandoned it years ago.
What to do
- Suppress immediately and never retry.
- Prune addresses that have not engaged in a long time — at Yahoo these become traps rather than merely going quiet.
- Use confirmed opt-in so an address is verified before it ever enters the list.
- Watch the rate. Unknown recipients feed the reputation that produces deferrals and then TSS09.
Questions
Why is this worse at Yahoo than elsewhere?
Because Yahoo reuses some long-dormant addresses as spam traps. A stale list does not merely bounce here — it can put you on a trap, which damages reputation far more than an unknown user does.
More from Yahoo / AOL
554 5.7.2554 5.7.2 [TSS09] — All messages from your IP will be permanently deferred (Yahoo)550 5.7.25550 5.7.25 — Forward-confirmed reverse DNS failed (Yahoo)421 4.7.0421 4.7.0 [TS03] — Temporarily deferred due to volume or complaints (Yahoo)