A few times a week I get email from some sorry Outlook/Exchange user which contains a dreaded winmail.dat file. Being a Thunderbird user, this presents a bit of a problem--one that has been well documented in Dealing with the winmail.dat file and unreadable attachments and How to Prevent the Winmail.dat File from Being Sent to Internet Users.

The are various "free" winmail.dat readers around, but this is 2008 not 1998. I shouldn't have to install the email equivalent of a "helper application" to read a fucking word doc that crappy email software couldn't encode in a sane format.

So anyway, I got one today and actually needed to read it. And I hadn't installed one of those stupid winmail.dat decoders since I had my laptop replaced. Faced with the prospect of actually installing software I wondered what'd happen if I forwarded a copy of the message to my Gmail account.

Well, wouldn't ya know it? The damned thing came through just fine. I was able to extract the attachment and open it faster than you can google "free winmail.dat decoder."

Kick Ass.

Just for kicks, I sent it to my dormant Yahoo! Mail account too... and it was also able to extract the Word document from the winmail.dat file.

Now why on earth hasn't this functionality been built into Thunderbird? Or Windows for that matter?

It's days like this that I might confuse my laptop for a stone tablet... just for a moment or two.

Posted by jzawodn at January 15, 2008 05:21 PM

Reader Comments
# David Ascher said:

Hey Jeremy -- I'm with you on that. I don't quite know the story about winmail.dat handling and why it's not part of the standard Thunderbird build, but that's the kind of stuff that I'm hoping to tackle with the next releases. That, and calendaring, and switching to using SQLite for indexing contacts and emails, and, and, and...!

In this case, I suspect the history is that Mozilla tries to carefully balance the usability benefit of adding the feature on one hand, with the "internet health" concern of enshrining support for a non-standards based format on the other. My call on this issue would be to follow the old robustness principle ("be conservative in what you do, be liberal in what you accept from others"), and hence add the feature to Thunderbird core.

As a funny coincidence, today I considered blogging asking why my iPhone IMAP reader can read Excel attachments but can't handle ".eml" attachments, which AFAICT are just RFC2822 of some sort.

on January 15, 2008 05:51 PM
# chris kim said:

The first line says it all: some "sorry" outlook user. yeah, like everyone on the planet, you mean? Sounds to me like the thunderbird user is the one who is sorry.. Outlook, IE and the rest always worked like a charm for me.

And going back to a statement I made a while back - I haven't seen a virus on any client system I support in over a year now..

IT Director, Windows guru.

on January 15, 2008 06:00 PM
# Jeremy Zawodny said:

Chris: no, not all Outlook users are sorry. But there are enough of them out there that I seem to bump into them on a routine basis.

on January 15, 2008 06:08 PM
# Phil Ringnalda said:

Judging by bug 77811, there's an extension at http://lookout.mozdev.org/ which reads them, that the author is willing to tri-license and contribute, once David hires someone with the time to work with the extension author to turn it into acceptable core code :)

(Not sure whether it's already tri-licensed, since Mozdev's CVS viewer seems to just want to tell me that my "IP Has Been Blocked From Mozdev ..." today.)

on January 15, 2008 06:20 PM
# said:

How did you do it? I had only the option to download the file in gmail. If I download it, it is still a .dat unreadable file...

on January 16, 2008 03:53 AM
# Matt. H said:

How about this interesting problem, which probably means that their is a problem with the Exchange setup at work.

Yesterday I replied to all to a message from an outside person and I added 3 people inside the company to the reply. Two of the people inside received a winmail.dat file instead of the message. To me that is really strange because the message didn't even leave exchange. We are all using OL2007 and the message was in HTML. We tried resending in RTF and TXT and they still couldn't get the message.

on February 9, 2008 07:44 AM
# Kris said:


I'm currently in the proces of moving all my Outlook mail (from a local .pst file) to my gmail IMAP account.

Many messages now have a winmail.dat attachment and when i click on it in Gmail, Firefox asks me what to do with it?

Maybe Internet Explorer has a built-in addon to open the winmail.dat?

on April 3, 2008 01:57 PM
# alissa Martin said:

Received one of these files from a client and for the life of me can't open it. Downloading Freeware like TNEF's Enough isn't possible due to the strict policy here -- no downloading apps & must have admin privileges to do so.

Tried forwarding to these accounts with no luck: .mac, aol, yahoo & G-mail
Forwarded it to my husband who uses Outlook and he couldn't open it either.

This client is notorious for freaking out if we have any issues with anything he sends and is not receptive to anyone telling him how to use his machine.

Found one site that you can upload the file to be decoded but it's not secure and i'm just not comfortable with that...

Any help? Why isn't gmail converting like others say it will?

on June 17, 2008 07:53 AM
# scrut said:

There's an add-on in thunderbird called 'Look out' that allows you to read .dat attachments.


on July 22, 2008 01:30 AM
# Bill said:

Hi all
I wonder whether the problem people have had with gmail is using it within Thunderbird.
I have had someone send me an attachment, which has arrived as a winmail.dat file.
I could not read it in the Thunderbird gmail folder, but when I opened the gmail program within Google, no problems with seeing the attachment as sent.
Thanks to scrut for pointing me to the Thunderbird addon! :)

on June 5, 2009 04:08 AM
# said:


on August 29, 2009 03:58 PM
# Thomas Staub said:

If everything fails you could use an online converter like http://www.winmaildat.com .

on January 12, 2010 02:13 AM
# Tom said:

I have been reading all the topics regarding .dat files. I haven't heard anyone with the exact problem I have.
When I send attachments to my brother and my son (so far they seem to be the only ones)the pdf or jpeg or whatever files are received as .dat files. This only seems to happen with MY emails. They receive similar files from other people without a problem.
I use outlook, they are on earthlink and comcast as ISP's.
Is it possible that I am being flagged as suspicious and their ISP is automatically changing the file extension?

on March 25, 2010 05:13 PM
# Kumiorava said:
on May 8, 2010 05:18 AM
# JohnC1990 said:

Thanks Jeremy for this useful info. It really worked for me! Though for my iPhone 4.0 I couldn't use this same method or any of the "free" winmail decoders, so I searched a bit and found this cheap but extremely useful app called "Winmail File Viewer" on the iTunes store which does exactly that, but on the iPhone! :)

on July 31, 2010 07:49 AM
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