Mac users receive winmail.dat attachments

What are winmail.dat attachments?
Users may receive this file as an attachment instead of the regularly attached file under these circumstances:
  1. The sender is using Outlook for Windows
  2. The sender has a contact record for the recipient enabled to use Rich Text or his default settings are set to use Rich Text
  3. The recipient is not using Outlook for Windows
Recipients of these files must use third-party software to decode the attachments or ask the senders to deliver the files using a different means. Non-Outlook users on both Mac and Windows systems may receive these winmail.dat attachments. It is not limited to just one platform.
RFT (not to be confused with the RTF format used as a markup language for text based documents) was developed by Microsoft before the use of HTML in email. Microsoft wanted to provide some way for users to send rich emails, complete with embedded graphics and formatted text. Microsoft developed its own far more capable, format and called it RFT. It was only ever implemented by Microsoft Exchange/Microsoft Outlook. Because it was a proprietary format, it could only be fully interpreted by other Microsoft Outlook clients. Other mail clients receiving such email received the formatting information and any embedded attachments in a winmail.dat file attached to a plain text version of the body of the email
How do I prevent sending winmail.dat attachments?
The sender using Outlook for Windows must make the changes plus you also need to delete your Auto Complete cache file.
Multiple locations within Outlook for Windows and Exchange can enable Rich Text settings, making eliminating this problem difficult.
  • The Rich Text setting may be specified in the Outlook for Windows contact record for the recipient.
  • Outlook also has a default setting, which may be enabled for Rich Text, that will apply to all newly created contact records.
  • An Exchange server, if used, may impose this as a default or impose this as an initial default setting.
To verify that a recipient’s contact record is not set to send Rich Text messages, open the contact record form the address book, double-click the E-mail address or right-click the E-mail address and select Outlook Properties… Set the Internet Format to either Let Outlook decide the best sending format or Send Plain Text only. This must be done for each contact record.
  • To set Outlook 2010’s default mail format, select Options from the File tab, select the Mail tab and select HTML or Plain Text from the Compose messages drop-down menu.
  • To set Outlook 2007 and earlier default mail formats, select Options from the Tools menu, select the Mail Format tab and select HTML or Plain Text from the Compose in this message format drop-down menu.
  • If you are still experiencing an issue deleted your cached email address from the auto-complete cache in Outlook as it will have a remembered format for addresses
  • On the exchange server you can override the other settings specific remote domains following these steps:
  • Open the Exchange management console and drill down to Organization Configuration/Hub Transport/Remote Domains.
  • Choose New Remote domain and add the domain name. Finish
  • Open the properties.
  • Go to Message Format tab
  • Set Exchange rich-text format to Never use