Observations :

Recent reported instances of users getting winmail.dat attachments in webmail wherein

• Users sometimes find that they receive mail with a strange file attached: winmail.dat
• Attachment in received mail will changed to winmail.dat (sender sent other type like pdf,xls)

Cause :

Outlook uses a proprietary email format that only Outlook and Exchange can decipher. When the recipient doesn't use Outlook, they get a plain text message and an attachment named winmail.dat.This causes problems, not for the sender of the email, but the recipient, particularly when actual files are attached to messages..

Generally Formatting options may cause these problems
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 that Outlook will use on the receiving end to display this email message correctly However other mail clients cannot decipher as a result a winmail.dat attachment is seen

Solution / Workaround :

One solution to this problem is to search for winmail.dat decoder on internet & use when such mails are received, this will at least allow you to decode the winmail.dat files. Other than this there’s not much you can do on your end to fix the problem, since it's not your email application generating the issue.

Similarly you can ask the sender to make changes in formatting at their end, as discussed in some links mentioned below:

• Why are outgoing attachments received as a winmail.dat file on the recipient's computer?
• Outlook is sending winmail.dat attachments.
• Why do some of my incoming emails contain a winmail.dat file?.