550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup
Exchange tried to resolve the recipient address against its directory and found nothing. It is the Exchange-flavoured "user unknown". When the sender is inside the same organisation the address simply does not exist; when the sender is outside, the recipient tenant's domain is authoritative in Microsoft 365 and the mailbox is not there.
What the server replies
550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient julia.stein@example.com not found by SMTP address lookup
- Sent by
- Microsoft 365
- Type
- PermanentRetry fails
- About
- Recipient problems
- Enhanced code
- 5.1.10
Who sends it
Exchange Online and on-premises Exchange transport, during recipient resolution, after the message was accepted. It arrives as an NDR rather than an SMTP rejection, which is what distinguishes it from the 5.4.1 edge block.
Why it happens
- The recipient was removed, or never existed.
- A hybrid setup where the mailbox is on-premises but the cloud copy of the directory does not have it.
- The domain is configured as Authoritative although some mailboxes are hosted elsewhere.
- An alias was removed from a user and old contacts still use it.
How to fix it
Verify with the recipient
External senders can only confirm the address and stop sending to it if it is wrong.
Admin: check directory sync
In a hybrid environment confirm the mail user exists in Exchange Online with the right proxy addresses. Run a delta sync if it was created recently.
Admin: switch to Internal Relay
If some addresses in the domain are hosted on another system, set the accepted domain to Internal Relay and add a connector to that system so unresolved recipients are forwarded.
Check it yourself
These free tools show the records and connections behind this code for your own domain or server.
Related codes
- 550 5.4.1 Recipient address rejected: Access deniedExchange Online looked the recipient up in the tenant's directory and did not find it, so it refused the address at RCPT TO.
- 550 5.1.1 User unknown / recipient address rejectedThe most common bounce there is: the mailbox does not exist.
- 550 5.1.1 The email account that you tried to reach does not existThere is no mailbox at that address on Gmail or on the Workspace domain.
Sources
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