Letter Opener is described as 'Open Winmail.dat, MSG and EML files. A new way to seamlessly view e-mail files without requiring an App download. Open E-Mail Files easily and securely'. There are four alternatives to Letter Opener, not only websites but also apps for Windows and Mac. The best alternative is winmaildat.com, which is free. Other great sites and apps similar to Letter Opener are Winmail.dat Viewer Pro Edition (Paid), Winmail.dat Reader (Free) and Winmail Opener (Free).
Read, view, open, convert and save the files on winmail.dat email attachments. Easy-to-use. Graphic interface (no command-line tool). Often you can obtain winmail.dat attachment in gmail and then you need this is tool.