[Editor’s note: The following review is part of Macworld’s GemFest 2009 series. Every day until the end of June 2009, the Macworld staff will use the Mac Gems blog to briefly cover a favorite free or low-cost program. Visit the Mac Gems homepage for a list of past Mac Gems.]

When using Apple Mail, if you’ve ever received an e-mail that included an attachment called winmail.dat, you know the frustration of not knowing what was sent to you. When this comes from an employer or client, it’s even more frustrating. You may have heard, “Oh, you’re on a Mac” with an audible sigh as if it’s your fault. The truth is, the issue is with Outlook or Outlook Express—even Windows users who don’t use Outlook can be frustrated by winmail.dat attachments.

That winmail.dat file may only be the rich text format (RTF) version of the message you’re already seeing as plain text, in which case all you’re missing is formatting that may not be important. But the file could contain an important graphic, calendar entry or invitation, task, address book contact, Microsoft Outlook Note, or nested messages. In those cases, you may miss important communications.

With Letter Opener in Apple Mail, the worry is gone. Letter Opener converts winmail.dat files on the fly, so you can actually see and use what the sender intended you to have. There’s no fuss with Letter Opener, no need to drag the winmail.dat file onto another application or call upon several programs to help you. Once Letter Opener is installed, you can forget about it.

You will barely notice Letter Opener working. If you don’t purchase a software license, you’ll see two unobtrusive lines of text above the From field that show that Letter Opener is at work for you. Once your register a license, you see nothing from Letter Opener, unless you check under the Mail menu. I actually prefer to see the notification so I know when I’m getting a winmail.dat file and that Letter Opener is doing its job.

Letter Opener quietly displays the fully formatted rich text message in your e-mail window, or translates the attachment to iCal or Address Book format for you to act upon. Mission accomplished.

If you regularly receive winmail.dat files from business associates, Letter Opener is well worth the investment.

Want to stay up-to-date with the latest Gems? Sign up for the Mac Gems newsletter for a weekly e-mail summary of Gems reviews sent directly to your inbox.

Our 186-page Mac Gems Superguide is a veritable greatest hits of Mac Gems. It's available as a PDF download, PDF on CD-ROM, or as a printed book. Learn more about the Mac Gems Superguide.

[Deborah Shadovitz is a Web designer and the author of Adobe GoLive 5 Bible (IDG Books, 2001). She also penned Mac Design’s GoLive column, and was a contributor to SBS Design.]