People that use Apple Mail for their email app will likely experience a problem from time to time, where emails they receive have a mysterious attachment named winmail.dat. This attachment can’t be opened, and any files that should have been attached to the email will be missing.

This winmail.dat problem is caused by Windows users, who are using Outlook for their email app, and are sending email using Rich Text Format (RTF). On the Windows side, Outlook sends RTF emails using Transport Neutral Encapsulation Format (TNEF) so that the message retains the same formatting for other Windows Outlook users. Unfortunately Apple Mail can’t deal with TNEF, and therefore dumps this data into the winmail.dat file.

Historically, a common fix for this issue was to have the Windows user configure their Outlook to send emails to Macs in either Plain Text or HTML format, instead of using RTF. Although this works, it isn’t always easy to walk the Windows user through how to make this configuration change because it varies depending on how their Outlook is setup.

I recently learned of a more straightforward fix for this problem. Microsoft released a hotfix that prevents Outlook 2007 and 2010 from sending RTF files using TNEF. If the Windows user applies this hotfix, it will prevent Macs from potentially receiving winmail.dat attachments. See more here for more details: http://support.microsoft.com/kb/958012