IT Talk – What’s with these mysterious “winmail.dat” files that show up in my email?

You've probably seen those "winmail.dat" a couple of times in your email inbox and asked yourself: (feel free choose your own 4-letter word) "What the heck is that?" and "How the heck can open it?" Winmail.dat files are an encoding method used by Microsoft Outlook when an email with attachments is routed through Microsoft Exchange Server. It usually contains the rich text format (RTF) of the body of the email, which has the information on fonts and formatting. BUT, it can contain other important information - such as a graphic, Outlook Calendar appointments and invitations, contact info or other messages. You wouldn't want to miss any of those.

Mystery Solved? Not quite - what do you do with it?

The best way to deal with "winmail.dat" files is to run it through a program like WMDecode. Just install the program, drag and drop the attachment and - open sesame - you'll get your decoded attachment!

For Mac Users (who are the most often plagued by these files) Letter Opener ($29.99 for a single user) is a program you can install and will open these pesky files instantly, and even convert Outlook files into iCal and Address Book formats.

And if you're an Outlook user, do your recipients a favor and turn it off (especially if not everyone in your address book is an Outlook user). Just go to Tools > Options and click on the Mail Format tab. For "Compose in the Message" Format, select HTML or Plain Text and for "Internet Format" under "When sending Outlook Rich Text messages to Internet recipients" choose either Convert to Plain Text or Convert to HTML.

About Brent Whitfield

Brent Whitfield is CEO of DCG Technical Solutions, Inc., which provides IT Support in the Los Angeles area since 1993. DCG exists to help our clients choose, implement, and manage IT and cloud solutions that are cost effective and reliable. DCG's pro-active approach to IT is ideally suited for companies who depend on reliable IT infrastructure, but don't want to spend a lot of money to keep it that way. DCG was recognized among the Top 10 Fastest Growing MSPs in North America by MSP Mentor. Brent has been featured in Fast Company, CNBC, Network Computing, Reuters, and Yahoo Business.