Microsoft (Outlook.com and Microsoft 365) · permanent error
550 5.7.134: mailbox rejects mail from outside the organization
The recipient mailbox is set to accept only authenticated senders, so mail from outside its organization is rejected.
- Microsoft
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550 5.7.134
- 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 5.7.134 Sender was not authenticated for mailbox
What it means
This is the mailbox version of the same restriction: the mailbox's message delivery restrictions require senders to be authenticated, which outside senders never are, so they get 550 5.7.134. Some mailboxes are set up this way on purpose. An admin of the recipient organization can clear Require senders to be authenticated, or keep it and add specific outside senders, represented by mail contacts or mail users, to the mailbox's allowed senders.
Common causes
- The mailbox has Require senders to be authenticated turned on.
- The mailbox is intended for internal mail only.
How to fix it
- 01Ask the recipient, by another channel, whether the mailbox should accept outside mail.
- 02Admins: in Recipients > Mailboxes, open Manage message delivery restriction and clear Require senders to be authenticated.
- 03Or add the outside sender as a mail contact and list it under the accepted 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 5.7.134" 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 5.7.133Group rejects mail from outside the organization.The recipient is a distribution group set to reject messages from senders outside its organization.
- 550 5.7.136Mail user rejects mail from outside.The recipient is a mail user set to reject messages from senders outside its organization.
Further reading
All 111 Microsoft codesEvery code in the wikiPaste your own bounce