In RFC 3463, 5.4.1 means "no answer from host", but Microsoft 365 (Exchange Online) reuses it for "Recipient address rejected: Access denied" — usually the mailbox does not exist or directory-based edge blocking is on.
Hard bounce · permanent
Common causes
- The mailbox does not exist in the recipient's Microsoft 365 tenant
- The recipient domain's MX points to Microsoft but the domain is not fully set up there
- The recipient blocks external senders for that address
How to fix it
- Confirm the address with the recipient
- If it is your own Microsoft 365 domain, check the user/alias exists and the domain is verified
- Remove the address if it keeps bouncing
MX CheckerRun the right check for your domain or paste the full bounce message for a precise explanation.
Open MX Checker →Frequently asked questions
Is 5.4.1 my fault?
Usually not — it is decided by the recipient's directory. Confirm the address through another channel.