What the receiver is telling you

Exchange's directory lookup found no object with that SMTP address. It is Microsoft's flavour of user-unknown, with the added possibility that the address exists but is not an accepted domain in the tenant — a company that added a new domain to Microsoft 365 but never made it an accepted domain sees every inbound to it fail with 5.1.10. For a cold sender the distinction does not matter: the address does not receive mail.

What a hard bounce means for your reputation

Receivers count hard bounces. Gmail and Microsoft both treat a sender whose mail is addressed to people who do not exist as a sender who did not get those addresses from the people themselves, which is the definition of unsolicited. The working threshold is two percent of a send: under that is noise, above it starts to cost domain reputation, and above five percent a fresh domain can be throttled after a single campaign. The fix is upstream — verify before sending — not downstream.

What to do

1. Remove the address.
2. If you know the person exists — a reply you received last week — the tenant may be mid-migration; wait a week and try once.
3. If the whole domain returns 5.1.10, the company's Microsoft 365 setup is broken on their side. Nothing you can fix.

don't Do not retry on a schedule. Exchange counts repeat attempts to unknown recipients as directory harvesting and rate-limits the sending IP.

Decode the whole bounce

Paste the complete failure notice — every code and the receiver's wording — and get each part explained together.

Common questions