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E-Mail and winmail.dat
By Martin English | February 25, 2008
Occasionally, I receive e-mails from users of Outlook/Exchange in plain-text messages with a large attachment at the end called either “Winmail.dat” or “ATT0001.dat”. This can occur for pretty much any email client that receives mail from Outlook Exchange. The reason why this happens is described fairly well in Microsoft Technote 241538, but here goes a fairly basic explanation.
The files – Microsoft TNEF stream files – are usually received by SMTP based e-mail programs from Microsoft Exchange and Microsoft Outlook users. The SMTP based e-mail program will usually receive either a MIME attachment named “winmail.dat” or a MIME attachment with the type “application/ms-tnef.”
The file is a rich text (or MAPI) message that is sent from Outlook to Exchange. When Exchange sends the message to an outside server it writes the MAPI message as a MIME attachment. The unfortunate side effect of this plan is if the Outlook user has someone in their address book as a person who can receive “Rich Text” then the user will receive the TNEF file whether the user uses Outlook or not.
In short, these attachments may (or may not) contain data that the sender of the email wants us to see.
A tool for reading TNEF format on Windows and Linux is the tool called tnef. It is released under the GNU General Public License (GPL). There are other alternatives for Windows, which I’ll discuss below. Installation of tnef on Windows is not trivial as it also requires installing the “Cygwin” tool first. Look at the instructions for installing TNEF on Windows or Linux before deciding whether this is appropriate for you
For Lotus users, NSF Tools has a fix that converts the TNEF ‘winamil.dat’ fo Lotus Notes users who are happy editing their mail agents.
A simpler alternative applicable on all Windows machines may be the winmail.dat reader. This is easy to install, and runs outside your mail client. You double-click on the winmail.dat attachment and this opens it in the winmail dat reader.
I also found references to Fentun’s tnef handling tool, a no-cost proprietary program. It appears from their website that it only works as a Windows 95 tool, so I haven’t investigated it further.
Note that there is also a fix available at the Lotus Notes / Domino server level as well, but this has some dependencies and issues, and can only be setup by the Lotus / Domino Administrator
Topics: Technology | 3 Comments »
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February 25th, 2008 at 4:19 pm
“functionality to decode winmail.dat attachments was added to Domino release 6.5.6 and 7.0.2.”
Technically its not a “fix” for the Domino server, rather an enhancement, as it is actually Outlook/Exchange that is broken!!
February 25th, 2008 at 8:26 pm
good point
Thanks for the correction
March 10th, 2008 at 2:15 am
For the Mac is the Apple Mail Plug-in OMiC available to fix this.