Permanent failure (hard bounce)
Email error 550 5.4.1: Recipient address rejected: access denied
The server refused the recipient before accepting the message. It is common with Microsoft 365 and other systems that check the address against a directory at the edge of the network.
Common causes
- The recipient address does not exist in that organisation.
- The organisation blocks outside senders for this mailbox or group.
- The address was deleted but the old address is still on your list.
- A directory-based filter treats unknown addresses as an attack.
How to fix it
- Treat the address as invalid unless you can confirm it with the person.
- Ask for a current address through another channel.
- Remove it from the list so it does not bounce again.
- Verify the list so addresses like this are caught before sending.
How to prevent it
Remove the address from your list and verify the rest before the next campaign.
Clean a whole list · MX lookup · SPF, DKIM and DMARC checker
Frequently asked questions
Is 5.4.1 the same as user unknown?
In practice yes: the system will not deliver to that address. The wording differs by provider.
Will retrying help?
No. It is a permanent rejection.
Other error codes
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