Microsoft 365 / Outlook
Microsoft 365 550 5.7.133 — "Sender not authenticated for group"
550 5.7.133
The recipient address is a group or distribution list that's set up to reject messages sent from outside its organization. Your message was refused because you're an external sender. Microsoft notes that only an email administrator for the recipient's organization, or the group's owner, can change this setting. Nothing about your domain or your authentication caused it.
How to fix it
- 1
If you're the sender
Email a person in that organization directly, or ask them whether the group should accept outside mail. Resending from another address won't help unless that address is inside the recipient's organization.
- 2
If you own the group: Microsoft 365 group
In the Microsoft 365 admin center or Outlook group settings, allow people outside the organization to email the group.
- 3
If you own the group: distribution list
In the Exchange admin center, open the distribution list's delivery management settings and allow senders inside and outside the organization. In PowerShell this is the RequireSenderAuthenticationEnabled setting on the group, set to false.
- 4
If internal colleagues get this bounce
Internal senders count as outside when they send from an address the tenant doesn't recognize as its own, for example a personal address or an alias of another domain. Have them send from their organization mailbox.
Meaning current as of 2026-10-24, cross-checked against the provider's own documentation. Providers do occasionally redefine codes — if this doesn't match what you're seeing, the source link is the authoritative reference.