KB ARTICLE: winmail.dat

If you receive a reject message regarding a winmail.dat attachment, the sender is using Microsoft Outlook email software.

A winmail.dat attachment is sent using "Transport Neutral Encapsulation Format" (TNEF), this is a non-standard email format not used today. Most mobile phones, Outlook and other email software cannot open winmail.dat attachments.

It is easily fixed, read on...

The reason?

While it is possible you may have the latest software version it is only the sending party that causes this issue.

The contact item in question (the recipient) in Outlook address book/contacts of the sender has a ‘hidden un-editable field’ not visible to the sender, this setting is historical to the sender’s Outlook contact entry only and this setting tells Outlook how to send the email to the specific recipient with the TNEF email encoding.

This encoding performed from the sender’s Outlook is non-standard so many new software items present this as a winmail.dat attachment.

Can you open winmail.dat attachments?

Download or use a third party online tool to read the winmail.dat email attachment manually, see: https://www.google.com/search?rls=en&q=winmail.dat

winmail.dat senders, read on to resolve the issue...

Method to fix 1, Sender re-creates contact (simple)

  1. Find the contact entry that the email was sent-to in the Outlook address book/contacts.
  2. Copy/write any details if you need to keep about the contact as you will need to keep/recreate later if required.
  3. Delete the contact from outlook address book/contacts.
  4. Re-send email to the recipient and confirm with recipient that the content/attachments are showing as intended.

Method to fix 2, Sender changes outlook settings

There are two options for disabling TNEF:

In at least Outlook 2002 (a.k.a. Outlook Xp) and Outlook 2003... On the "Tools" menu, click "Options", then click the "Mail Format" tab, and then the "Internet Format" button. Set "When sending Outlook Rich Text messages to Internet ..." to either "Convert to HTML format" or "Convert to Plain Text format".

In all versions of Outlook, you can disable TNEF completely: On the "Tools" menu, click "Options", and then click the "Mail Format" tab. In the "Send in this message format" list, click "Plain Text" or "HTML", and then click "OK".

Method to fix 3, Sender Exchange server administrator makes a change

See: https://technet.microsoft.com/en-us/library/bb310786%28v=exchg.150%29.aspx?f=255&MSPPError=-2147217396