Symptoms:

Users in external companies receive an attachment named winmail.dat though it is not included by the sender.

Cause:

Microsoft Office Outlook uses a message format encapsulation known as TNEF (Transport Neutral Encapsulation Format) which is natively used for storing and displaying information like richly formatted text, embedded objects (OLE), certain outlook features like forms, voting buttons, etc. This format is understandable to Microsoft Office Outlook and Microsoft Exchange Server both.

Many messaging sub systems are not capable of translating this format into their display formats and assume it as additional information attached along within the email. This issue has been identified to be occurring with messages sent using RTF and HTML format and when Microsoft Office Outlook is used as an email client.

Resolution:

If you have Microsoft Office Outlook configured with Microsoft Exchange Server and recipient outside your company complain that they always receive an additional attachment with every email from you, you should send all email messages to these recipients using Plain Text Format.

The same resolution applies to a locally created distribution list in individual user mailboxes.

For individual email addresses:

1.     Double click on email address that you have typed in To: field of a new message or being replied/forwarded message.

2.     Select “Send Plain Text Only” from the Internet Format in properties window.

For local distribution lists:

Sending format can not be applied to a distribution list that resides in user mailbox directly. If you already have a distribution list created you will need to manipulate the sending format for every recipient in the DL.

External Links:

Winmail.dat attachments are included in received e-mail messages in Outlook