Why Do E-Mail Messages Come With Winmail.dat Files? – Support Center Ticketing

We've all seen mysterious .dat file attachments in our inboxes from time to time. In most cases the .dat attachment originates in Microsoft Outlook, which produces outgoing messages using a slightly modified version of the Rich Text Format. This action helps to preserve fonts, formatting, and the like, but it often causes problems for the recipient. If your e-mail program does not support this particular blend of RTF, you'll end up with that danged .dat attachment--and no easy way to open it.

You have a couple of options. First, you can ask the sender to turn off Outlook's RTF setting in favor of plain-text e-mail and then resend the message. Second, you can try opening the attachment in Notepad. You may see a lot of garbled-looking code in there; but if you comb through it, you should be able to find the core text. Finally, try WMDecode, a free utility that will scan the Winmail.dat file for attachments (and only attachments, not the formatted version of the e-mail) and save the items along with their original filenames.