550 5.7.1: public folder does not accept external mail
An external sender wrote to a mail-enabled public folder in Microsoft 365, which by default accepts mail only from authenticated senders.
- Microsoft
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550 5.7.1
- 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 5.7.1 RESOLVER.RST.AuthRequired; authentication required [Stage: CreateMessage]
What it means
Mail-enabled public folders in Exchange Online refuse mail from external senders by default, to protect them. An outside sender writing to one gets 550 5.7.1 RESOLVER.RST.AuthRequired. The sender can do nothing about it. An admin of the receiving organization can let the folder accept external mail, either by turning off the requirement that all senders be authenticated in the folder's mail flow settings, or by giving the Anonymous user the CreateItems right on the folder.
Common causes
- The public folder requires all senders to be authenticated, which is the default.
- The Anonymous user has no permission to create items in the folder.
How to fix it
- 01Ask the recipient organization's admin to open the folder to external senders.
- 02Admins: in the Exchange admin center, open the public folder's mail flow settings, clear Require that all senders are authenticated and select All senders.
- 03Or grant Anonymous the CreateItems right with Add-PublicFolderClientPermission -User Anonymous -AccessRights CreateItems.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Error when external users send mail to mail-enabled public folderslearn.microsoft.com
- Fix NDR error "550 5.7.1" in Exchange Onlinelearn.microsoft.com
Related codes
- 550 5.7.135Public folder rejects mail from outside.The recipient is a mail-enabled public folder set to reject messages from senders outside its organization.
- 550 5.7.13No permission to post to the public folder.Delivery to a mail-enabled public folder failed because the permission the sender falls under does not allow creating items in it.
5.7.1 at other providers
- 550 5.7.1Blocked by a domain policy.A rule set by the recipient's or the sender's Google Workspace admin blocks this message, so Gmail rejected it under that policy.
- 550 5.7.1Relay IP does not match the sending domain.Google's SMTP relay knows your IP, but the mail's sending domain is not one that IP is registered to relay for in your Workspace account.
- 550 5.7.1Blocked as likely spam.Gmail's filters judged this message likely to be spam and rejected it outright, instead of delivering it to the spam folder.
- 550 5.7.1IPv6 sender without PTR or authentication.Gmail rejected mail that arrived over IPv6 because the sending address lacks a matching PTR record, or the message failed authentication.
- 550 5.7.1Blocked for low IP reputation.Gmail permanently rejected the message because the IP it came from has a very low reputation with Gmail.
- 550 5.7.1Blocked for low domain reputation.Gmail permanently rejected the message because the domain it comes from has a very low reputation with Gmail.
Further reading
All 111 Microsoft codesEvery code in the wikiPaste your own bounce