What the receiver is telling you

You are sending through Microsoft 365 with a From address on a domain that is not an accepted domain in the tenant. Microsoft refuses outbound for domains it cannot verify you own. This appears when a cold email tool is connected to a 365 inbox and set to send from a different domain, or when an alias was added to a mailbox without the domain being added to the tenant, or when a domain was removed from the tenant while mail was still queued for it.

Why a policy block must not be treated as a bad address

Sending tools file every 5xx under "bounced", and the natural reflex is to scrub the address. For a policy block that is exactly wrong. The recipient exists and would have received the mail; the receiver refused it because of your domain, IP, authentication or content. Scrubbing throws away a lead and leaves the cause in place, so the next send produces the same block against the next thousand addresses. The right response is to stop sending from the affected domain, find what the receiver named, fix it, and resume at low volume.

What to do

1. Add the domain to the Microsoft 365 tenant and verify it with the TXT record, then add the alias.
2. Or set the sending tool to use the mailbox's real primary address.
3. If the domain was removed deliberately, update every inbox and tool that still references it.

don't Do not try to relay around it through a third-party SMTP with the same From; DMARC will fail instead.

Decode the whole bounce

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

Common questions