Messages aren't evenly distributed if BasicAuth or BasicAuthRequireTLS is used in Exchange Server 2019, 2016, or 2013

Original KB number:  3195087


Consider the following scenario:

  • You create a receive connector to use Basic Authentication or Basic Authentication over TLS.
  • You don't use Anonymous Users as a permission group on this connector.
  • You send email messages to the Microsoft Exchange Front End Transport Service.

In this scenario, the mail flow always seems to proxy the message to the same Transport Service regardless of which Front End Transport Service the message comes in on. The mail queue database can unexpectedly grow on the SMTP server. This causes more resources to be consumed because of the additional messages that are routed to this server.


This behavior is by design. When you require Basic Authentication or Basic Authentication over TLS, these messages are authenticated and then are routed to the host of the active mailbox database copy where the authenticated user is located. If no mailbox exists for that user, the organization's arbitration mailbox is used.

Workaround 1

Disable the Client Proxy Prefer Mailbox Mounted Server by setting its value to "false." This distributes the load to other servers in the database availability group (DAG). To do this, follow these steps for all smart hosts in the send connector:

  1. Locate the MSExchangeFrontEndTransport.exe configuration file.


    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.


    When randomization is used, the server selection of a different Active Directory site can be used. The random servers used are only in the DAG of the active mailbox database copy and servers containing active database copies. If the active database isn't part of a DAG, only that server will be used.

Workaround 2

Don't use Basic Authentication on the receive connector. Instead, use an external secure authentication method.


This will make the server an open relay type. This is not a recommended configuration. For more information, see Allow anonymous relay on Exchange servers.

Workaround 3

Send messages to the Microsoft Exchange Transport Service directly. Do this by sending the mail to port 2525 or to another port the service is listening on. It won't do any load balancing at this point. The server the message is sent to is the server that processes the message. We recommend that you create a new receive connector to allow those connections coming from the SMTP source.