Winmail.dat Reader

Winmail.dat attachments in e-mail sent from Outlook

Recipients who use other e-mail programs sometimes receive a small attachment called winmail.dat with your message, and the files you attached may be missing or hidden inside it. This page explains why it happens and how to stop Outlook from sending it.

Why it happens

When a message is sent in Outlook's Rich Text format, Outlook packs the formatting, and often the attachments, into a TNEF container and attaches it as winmail.dat. Only Outlook and Exchange understand this container; other programs show it as a file they cannot open. Messages in HTML or plain text format never produce it.

How to prevent it

Change the default message format

1. In Outlook, open File > Options > Mail (Outlook 2007 and earlier: Tools > Options > Mail Format).
2. Set Compose messages in this format to HTML or Plain Text.
3. Set When sending messages in Rich Text format to Internet recipients to Convert to HTML format or Convert to Plain Text format, never to Send using Outlook Rich Text format.

Change the setting for one recipient

1. Open the recipient's contact in the People (Contacts) folder and double-click the e-mail address.
2. In the E-mail Properties dialog, set Internet format to Send plain text only.
3. Save the contact.

The setting of a contact overrides the default, so check it first if a recipient still gets winmail.dat after you changed the default. An old entry in the AutoComplete list can also keep the previous setting; delete it and type the address again.

Change the format of a single message

1. In the new message, open the Format Text tab and choose HTML or Plain Text.

If you are the one receiving winmail.dat files, you cannot fix them on your side: ask the sender to apply these changes, or open the file with Winmail.dat Reader.

Based on the Microsoft article: support.microsoft.com/kb/278061