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Why am I receiving winmail.dat attachments?

Users may receive this file as an attachment instead of the regularly attached file under these very specific 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.

Barry Wainwright, a long time Macintosh user and manager of the IT group for his company, posted this explanation on the YouTalk mailing list:

RFT (not to be confused with the RTF format used as a markup language for text based documents) was developed by Microsoft long, long ago, before the use of HTML in email became ubiquitous. Microsoft wanted to provide some way for users to send rich emails, complete with embedded graphics and formatted text. Remember that in those days (around e early nineties), all email was plain text only! Because there were no widely accepted and used standards in those days, each mail client did its own thing.

There were a few different solutions, Eudora had its own mark-up format, Setext was also developed and was most notably used by the TIDBits newsletter (I believe the setext edition was only recently discontinued). Microsoft developed its own far more capable, format and called it RFT. It was only ever implemented by Microsoft Exchange/Microsoft Outlook. It allowed the user to send mail containing formatting and embedded images and other documents in a way that seems so simple today, but was revolutionary some 15-20 years ago.

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 (sometimes, depending on the configuration of e mail client and server, this plain text version is missing). This was not a huge issue, because the use of Exchange was pretty much limited to enterprise level businesses, and was pretty much a standard between them, so most users of Outlook were communicating with other users of Outlook almost exclusively.

Non-Outlook users on both Mac and Windows systems may receive these winmail.dat attachments. It is not limited to just one platform.

How do I prevent receiving winmail.dat attachments?

The sender using Outlook for Widows must make the changes plus you also need to delete your Auto Complete cache file. (see below**)

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.

**Delete Auto-Complete cache file:

I checked everything you needed to check on the sending Windows PC and it was set up to use HTML as the sending format and my contact was set as ”Let Outlook decide…” But I was still receiving winmail.dat files from this sender. I tried deleting my contact record from the sender’s address book entirely and that didn’t fix it. Then I deleted my cached email address from the auto-complete cache in Outlook and that fixed it. [Added January 15. 2013. Contributed by Bill Tepper]

To enforce mail standards for Outlook for Windows within an Exchange environment, administrators should look to implementing Administrative Templates for Outlook via a Group Policy.

How can I view winmail.dat attachments?

Consider using Josh Jacob’s TNEF’s Enough, a winmail.dat decoder. It is available for Mac OS X 10.4 through 10.6 and is donationware.

Klammer allows you to open MSG & Winmail.dat files on Mac. $5.99 at Mac App Store.

Letter Opener Lite  $6.99 at Mac App Store. To get the integrated plugin version you need to purchase Letter Opener Pro directly for $29.99


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