If you use non-Microsoft software to read your email, chances are you eventually will receive a message that appears to include an attached file named winmail.dat. This occurs when the sender uses a Microsoft email client with settings that are incompatible with those of the recipient’s client software.
The message, including its attachments, can often be read without difficulty when using a Microsoft email client. This is due to Microsoft software being able to decode the message’s TNEF encoding. Non-Microsoft clients typically lack this ability, as TNEF is proprietary to Microsoft. The issue does not indicate a fault with the recipient's email client or with any of the email servers involved.
First, evaluate whether the issue actually causes loss of relevant information. If, on the other hand, the winmail.dat file constitutes but a cosmetic fault, you might decide simply to ignore it.
If the issue does need to be remedied, the sender may experiment with various settings in his or her email client until a combination is found that results in messages usable to the recipient. As adjusting the most obvious settings may not be enough, this task can daunt even an experienced user.
Microsoft has published knowledge base articles on this phenomenon. The article ‘How e-mail message formats affect Internet e-mail messages in Outlook’ includes technical information that may be useful in troubleshooting the issue. ‘Email received from a sender using Outlook includes a Winmail.dat attachment’ is another, much briefer article; unfortunately, it is also an over-simplification that often does not resolve the problem.
Various third-party software tools promise to decode winmail.dat attachments. Caveat emptor.
Should all else fail, you may decide to use a Microsoft email client (such as Outlook) to read the affected messages.