FAQ | Frequently Asked Questions

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Q: What's with this winmail.dat attachment? And why won't it open?
A: The winmail.dat attachment, or sometimes win.dat, is an “ms-tnef” filetype.

This is a proprietary e-mail attachment format used by Microsoft Outlook and sometimes Microsoft Exchange Server to send formatted e-mail messages. Almost nothing else can read these files. TNEF attachments can contain security-sensitive information, and they typically violate various Internet mail standards.

Just let it go.

You can look up tnef in wikipedia for more gory details: http://en.wikipedia.org/wiki/TNEF

Passing this information on the the original sender is probably typically useless because within the Outlook email client that probably sent the original message the TNEF encoding cannot be explicitly enabled or disabled.

Update 3-Nov-2010: It does look like one can now turn it off in some modern clients, and also at the server under some conditions.

  See: http://support.microsoft.com/kb/290809

On May 11, 2011, at 2:19 PM, Paul Waterstraat wrote: Greetings TSP, When people using uConnect and Outlook send me e-mail messages with an attachment the attachment arrives as the dreaded "winmail.dat" file instead of the original file. It doesn't seem to matter if the sender is using Outlook on a PC or Outlook 2011 for Mac.

On May 11, 2011, at 2:34 PM, Mike Waid wrote: Hi Paul, This has been a useful app for me for the few people who still send me RTF encoded emails: http://www.macupdate.com/app/mac/12705/tnefs-enough

On May 11, 2011, at 2:23 PM, Jackie Simmons wrote: NTEF issues. For Macs, http://www.joshjacob.com/mac-development/TNEF2.2.zip For pc thunderbird users. https://addons.mozilla.org/en-us/thunderbird/addon/lookout/

On May 11, 2011, at 3:03 PM, Paul Singh wrote: Paul: Windmail.dat attachments occur when a non-Exchange system receives an rich-text format (RTF) email message. There are three places where an RTF message can be generated: 1) Actual settings on the Exchange server as Sudhir points out. This, of course, is correctly configured within uConnect. We send millions of pieces of email with attachments every week, internal to uConnect, external to campus and to the world. 2) The sender chooses a recipient from their local address book/contacts (not GAL) and this recipient is setup as a MAPI recipient (able to receive RTF messages) when in reality they are not MAPI-recipients. This setting will override the setting on item 1 above. 3) Within the sender's email client, their default email settings is to not send messages as plain text nor HTML format. You can read additional details here: http://support.microsoft.com/kb/138053

On May 11, 2011, at 3:32 PM, Paul Singh wrote: > Recipients having to deal with converting attachments doesn't bode well for > the prospect of moving to uConnect. But maybe it's an excuse for me to > resurrect my BINHEX encoder for sending attachments. > - Paul. Hi Paul, In response to this comment, this message has an attachment, so it's from me (uConnect and Outlook 2010) to you (not on uConnect, and using whatever email client). I'm sure you did not receive a windmail.dat file. So as you can see...it's not uConnect that is the issue here. Regards, Paul Singh Attachment icon for winmail.dat