Bounce error fix

550 5.4.1: recipient address rejected, access denied

Microsoft 365 rejected your email because the address doesn’t exist at that company. Here is what to do, as the sender and as the admin.

What the bounce says

550 5.4.1 Recipient address rejected: Access denied. AS(201806281)

Despite the wording, this isn’t a block on you. It comes from Microsoft 365, which checks every incoming email against the receiving company’s list of real addresses. When the address isn’t on that list, Microsoft rejects it with this error.

If you sent the email

Check the spelling of the address in the bounce. One wrong letter is enough.

Assume the person has left, or that the address was guessed. Remove it from your list.

Verify lists before you send. Guessed and old addresses cause most of these bounces, and a high bounce rate hurts your domain. See soft bounce vs hard bounce.

If you run the receiving Microsoft 365

Check the mailbox or alias exists in Microsoft 365 with exactly that address.
Everyone at the domain bouncing? In the Exchange admin centre, open Mail flow › Accepted domains, switch the domain from Authoritative to Internal relay, save, then switch it back to resync it.
Hybrid setups: a newly synced mailbox can take up to 24 hours to be recognised.
Signature or filtering services that route mail out and back in can trigger it too; check their Microsoft 365 setup guide.

Make sure the rest is right

If addresses are real and you still bounce, check your SPF, DKIM and DMARC.