Unfortunately, Outlook does not "play nice" with the other email programs all the time. This causes problems, not for the sender of the email, but the recipient, particularly when actual files are attached to messages.

When an Outlook user composes and sends a message using either Rich Text Format or HTML Format, Outlook automagically generates a file, winmail.dat, and attaches it to the end of the message. winmail.dat contains formatting information, in a human-unreadable form, that Outlook will use on the receiving end to display this email message correctly. Unfortunately, Outlook is the ONLY email client program that can use this information! Netscape Messenger, Eudora*, and other email client programs don't understand this information.

Theoretically winmail.dat could also contain attachments, but myexperience has been that if other attachments (e.g. word documents, pictures) are included, they show up individually not in a winmail.dat file.

How to avoid sending winmail.dat:
If you are sending messages to a mailing list or as a group mailing, you MUST set ALL users up so that they receive plain text email. If even one user is set up in your address book, or your default setting is to receive Rich Text Format or HTML format email, everyone will receive that format.

If you enter the recipient's address manually in the To: line of your email message, you must choose Format->Plain Text from the menu bar or change the default to plain text in Tools -> Options.

There are several programs that allow you to decode winmail.dat
For Windows:
WWMdecode at Biblet.freeserve.co.uk

For Mac:

Links:
Email Ettiquette