Hi,

We have Exchange 2013 installed, and we receive emails sent from external senders normally. However, if an email is sent to a number of addresses, but one address is wrong, then server rejects complete email (nobody receives ) with response 550 5.1.1 User unknown.

Thank you for any help

I don't believe that is normal behavior for Exchange 2013.  Do you have any message hygiene appliances or services between the Internet and Exchange?

Hi,

We use Exchange Online Protection for message hygiene, but I can see in receive connector logs that message is entering server. I don't know how Exchange Online Protection and Mail Server communicate in this aspect.

• Edited by Qaku 21 minutes ago

This is the expected behaviour.

https://technet.microsoft.com/en-us/library/bb123891%28v=exchg.150%29.aspx

"If the inbound message contains a recipient that doesn't match any recipients in Recipient Lookup, the Exchange server sends a 550 5.1.1 User unknown SMTP session error to the sending server."

My emphasis on the "a".

Basically a single bad recipient will cause the entire message to be dropped.

Hi,

We use Exchange Online Protection for message hygiene, but I can see in receive connector logs that message is entering server. I don't know how Exchange Online Protection and Mail Server communicate in this aspect.

• Edited by Qaku Sunday, August 23, 2015 7:05 AM