550 5.7.136: mail user rejects mail from outside
The recipient is a mail user set to reject messages from senders outside its organization.
- Microsoft
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550 5.7.136
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The recipient's side. Their mail admin has to change something; you can tell them.
- What the server says
550 5.7.136 Sender was not authenticated
What it means
A mail user is a recipient in Exchange Online whose mail is delivered to an email address outside the organization. Like mailboxes and groups, it can require senders to be authenticated, and when that is on, an outside sender gets 550 5.7.136. Only an admin of the recipient organization can change it, in the mail user's message delivery restrictions: allow all senders, or keep the restriction and add particular external senders, represented by mail contacts or mail users, to its allowed list.
Common causes
- The mail user has Require that all senders are authenticated turned on.
- The address is meant to receive internal mail only.
How to fix it
- 01Contact the recipient by another channel and ask whether the address should take outside mail.
- 02Admins: in Recipients > Contacts, edit the mail user and, under message delivery restrictions, clear Require that all senders are authenticated and choose All senders.
- 03Or choose Only senders in the following list and add the external senders.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Fix NDR error "550 5.7.136" in Exchange Onlinelearn.microsoft.com
- Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
Related codes
- 5.7.12Recipient accepts mail only from its organization.The address you wrote to takes mail only from inside its own organization, so outside senders are refused until that organization's admin changes the setting.
- 550 5.7.134Mailbox rejects mail from outside the organization.The recipient mailbox is set to accept only authenticated senders, so mail from outside its organization is rejected.
Further reading
All 111 Microsoft codesEvery code in the wikiPaste your own bounce