Messages aren't evenly distributed when BasicAuth or BasicAuthRequireTLS is used in Exchange Server 2013 or later


Consider the following scenario:
  • Your mailbox is located on Microsoft Exchange Server 2013 or a later version of Exchange Server. 
  • Some messages that are sent to you through a custom send connector from another SMTP server (such as an Edge server) have the Smart Host Authentication Mechanism set to either BasicAuth or BasicAuthRequireTLS.
  • These messages target a single server in the destination or a set of servers in a database availability group (DAG).
In this scenario, the mail queue database unexpectedly grows on the SMTP server, and more resources are consumed because of the additional messages that are routed to this server. 


By design, all authenticated messages are routed to the host of the active mailbox database copy where the authenticated user is located. 


To work around this issue, disable the Client Proxy Prefer Mailbox Mounted Server by setting its value to "false." This distributes the load to other servers in the DAG. To do this, follow these steps for all smart hosts in the send connector:
  1. Locate the MSExchangeFrontEndTransport.exe configuration file.

    Note By default, this file can be found in the following location:
  2. Add the following line under <appSettings>:

    // Add the following line.
    <add key="ClientProxyPreferMailboxMountedServer" value="false" />
    // End of the added line.
  3. Save the changes, and then restart the Front End Transport service on the server.

Article ID: 3195087 - Last Review: Oct 3, 2016 - Revision: 1