Article ID: 940348 - View products that this article applies to.
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In Hosting Messaging and Collaboration Solution (HMC) 3.5, there is a Simple Mail Transfer Protocol (SMTP) Event Sink (Smtpdomx.dll) to handle large number of SMTP domains. This sink is installed on all front-end servers that are running Microsoft Exchange Server to handle incoming SMTP traffic. Sometimes, you receive the following error 550 message if you have lots of hosted domains:
Usually, this message only happens for the most recently added domains.
Unable to relay
This problem occurs because the Event Sink does not add an exclusive write-lock on the Domains.dat file when the Event Sink reads the Domains.dat file. In this case, the Event Sink does not finish reading the Domains.dat file because HMC 3.5 is updating that file to the new Domains.dat file version. And, HMC 3.5 only checks the Domains.dat file before the Event Sink accesses the Domains.dat file. Therefore, the Event Sink only reads a partial number of the domains from the Domains.dat file. Then, you receive an error 550 message if the Event Sink cannot handle an SMTP domain.
Hotfix informationTo resolve this problem, apply the hotfix that is described in Microsoft Knowledge Base article 931524. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
(https://support.microsoft.com/kb/931524/ )Hosted Messaging and Collaboration version 3.5 users receive a "550 5.1.1 User unknown" NDR message
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.