Hello everybody. I just installed SBS 2011 server with 10 users. I configured 3 accepted domains for Exchange: mes.local (authoritative), mes.com (internal relay) and ems.com (internal relay). I also unchecked "Block messages sent to recipients that do not exit in the directory" in Anti-Spam/Recipient Filtering. Most of emails are dilivered fine. The only time users receive the 550 5.1.1 error when the email is forwarded between 2 relays: for example user1@ems.com forwards to user2@mes.com. If account is changed in Outlook from ems.com to mes.com before sending then no error happens. Any ideas how to fix the problem? Many thanks, Oleg.

Why these domains are configured as internal relay domains? If the SBS server hold users of all these domains, it should be configured as authoritative. Shaba

See the screenshot. Internal Relay Domain is used when you have another messaging environment apart from the Exchange organization. From your first post, I don't see that your organization is having a second messaging infrastructure. Please clarify.Shaba

Both ems.com and mes.com are hosted by 2 different ISPs and we'd like to keep them for spam filtering and redundancy purposes.

Are you sharing the domains ems.com and mes.com between the SBS and the ISP messaging infrastructure? Shaba

I'm not sure what sharing mean in this case. I have MX record with lower priority number setup with the ISP.

Is there any user on your SBS server with ems.com or mes.com email address?Shaba

I also just changed both accepted domains to be authoritative but still got 550 5.1.1 error when tried to email a message between ems.com and mes.com.

I am sure that its not going to be authoritative. Authoritative means these domains ems.com and mes.com is hosted on your exchange infrastructure. It should be either Internal relay domain or External relay domain. For that to confirm, You need to tell me if any user on SBS is having the email address ems.com/mes.com. Shaba

All of my users have mes.local, mes.com and ems.com emails. The name can be different though: for example johnsmith@mes.local, info@ems.com and inquires@mes.com.

Ok, then internal relay should be the one. Do you have respective connectors configured? Please have a look on the below technet article. http://technet.microsoft.com/en-us/library/bb676395.aspxShaba

I created 2 New Send Connectors per tech article (using the same Smart Host), but again got the error 550 5.1.1 ... User unknown

hi, >>>If account is changed in Outlook from ems.com to mes.com before sending then no error happens. What's the primary mail address of the user? You can see his primary address in his mailbox email address tab. It will be marked in bold. Which user do you forward to,what' s his mail address? Use the message tracking log to search the mail, and then find more information about your message. hope can help you thanks, CastinLu TechNet Community Support