550 5.2.0: external contact has no valid primary address
Exchange Online could not deliver to an external contact because its primary SMTP address is missing from its proxy addresses or invalid.
- Microsoft
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550 5.2.0
- 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 sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
550 5.2.0 RESOLVER.ADR.BadPrimary; recipient primary SMTP address is missing or invalid
What it means
Mail contacts in Exchange Online stand for addresses outside the organization. When an Exchange Online user writes to one and gets 550 5.2.0 RESOLVER.ADR.BadPrimary, the contact's primary SMTP address was not added to its proxy addresses, or a legacy proxy address is set on it. The address the user picked may look fine; the problem is in the contact object, so an admin has to correct it. In hybrid setups the on-premises contact object, synchronized to the cloud, must carry the correct address.
Common causes
- The primary SMTP address was not added to the external contact's proxy addresses.
- A legacy proxy address is set on the contact.
How to fix it
- 01Delete the contact from the Outlook Auto-Complete list and try again.
- 02Admins: in the Exchange admin center, open the contact and check that its external email address exists and is formatted correctly, for example SMTP:adam@contoso.com.
- 03In a hybrid deployment, correct the proxy address on the on-premises contact and run directory synchronization.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 554 5.2.0Clutter folder missing in the recipient mailbox.Delivery to one Exchange Online mailbox fails because Clutter is turned on for it but its Clutter folder is missing.
- 5.2.0Mailbox problem, unspecified.The recipient's mailbox exists, but something about it stopped the delivery and the receiving server did not say what.
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