Configure Exchange Server to send RTF messages in HTML format

Optemail requires the Microsoft Exchange Server to act as a SmartHost. When it does this Exchange's default settings mean that attachments will be sent as winmail.dat files. The following article explains the problem in more detail and goes on to explain how to resolve this issue with a single configuration change in Microsoft Exchange Server.
Problem: Mail attachments are turning to winmail.dat

When an end user sends mail to the Internet from an Exchange Windows or Outlook client, a file attachment called Winmail.dat may be automatically added to the end of the message if the recipient's client cannot receive messages in Rich Text Format (RTF). The Winmail.dat file contains Exchange Server RTF information for the message, and may appear to the recipient as a binary file. It is not useful to non-Exchange Server recipients.

E-mails are sent outside the organization in the RTF format very rarely. If a user writes an RTF message in Outlook and sends it to an external address, Outlook will transform it to HTML by default (although the option may be disabled to prevent Outlook from converting the message format) and a disclaimer will be added to such a message. In Outlook 2013 you can find this option in Options -> Mail, section Message format:

Resolution: Configure Exchange's rich text format setting

It is possible to configure Microsoft Exchange so that messages in RTF are never sent outside via Exchange Server, follow these steps:

Exchange 2000/2003

Exchange 2007/2010

Exchange 2013/2016/Online

Exchange 2000/2003

  1. Run the Exchange System Manager program.
  2. In the left panel go to Global Settings | Internet Message Formats | Standard.
  3. In the context menu choose Properties and go to the Advanced tab.
  4. Set the Exchange rich-text format option into Never use.

5. Restart the 'Microsoft Exchange Transport' windows service in the services snap-in

Exchange 2007/2010

 Run the Exchange Management Console.

  1. Open Organization Configuration and then Hub Transport.
  2. Open Remote Domains.
  3. Select Properties:
  4. In the opened window please select:
    On Exchange 2007: Format of original message sent as attachment to journal report tab
    On Exchange 2010: Message Format tab
  5. Find section Exchange rich-text format and select Never use and hit Apply

6. Restart the 'Microsoft Exchange Transport' windows service in the services snap-in

Exchange 2013, Exchange 2016 and Exchange Online

  1. Open Exchange Management Shell.
  2. To see you current settings please run the following command:
    Get-RemoteDomain | Where {$_.TNEFEnabled -ne $false}

  3. For each Remote domain, please run the following command:
    Set-RemoteDomain -Identity Default -TNEFEnabled $false
  4. Restart the 'Microsoft Exchange Transport' windows service in the services snap-in

After configuring the Exchange server in this way, messages sent by users in RTF will be converted into HTML by the server.

Additional resources from Microsoft's Website.

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