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Transport Neutral Encapsulation Format (TNEF)

Transport Neutral Encapsulation Format (TNEF) is a standard used by Microsoft Outlook and Microsoft Exchange Server to store rich e-mails. E-mails in TNEF format can have audio and video attachments, voting buttons, colored text, calendar appointments and forms in them.

TNEF information is transferred via a file called winmail.dat or sometimes win.dat which is attached to an original message. E-mail message itself will only contain plain text version while winmail.dat will contain all advanced features. Winmail.dat file is transferred using application/MS-TNEF MIME type.

TNEF format is a binary file format. Microsoft Outlook and Microsoft Exchange Server use TNEF Reader component to read TNEF data and TNEF Writer component to write it.

Below is a list of all types of messages supported by TNEF messages. Message class is encoded in attMessageClass attribute and could be any of the following:

  • IPM.Microsoft Mail.Note – e-mail with a note
  • IPM.Microsoft Mail.Read Receipt – e-mail with a read receipt
  • IPM.Microsoft Mail.Non-Delivery – e-mail with a non-delivery notification
  • IPM.Microsoft Schedule.MtgRespP – e-mail with a positive meeting response
  • IPM.Microsoft Schedule.MtgRespN – e-mail with a negative meeting response
  • IPM.Microsoft Schedule.MtgRespA – e-mail with a tentative meeting response
  • IPM.Microsoft Schedule.MtgReq – e-mail with a meeting request
  • IPM.Microsoft Schedule.MtgCncl – e-mail with a meeting cancellation
Each e-mail message has an attribute with e-mail status. Values are listed below:
  • fmsRead – e-mail read
  • fmsModified – e-mail modified
  • fmsSubmitted – e-mail submitted
  • fmsLocal - unsent e-mail
  • fmsHasAttach – e-mail with an attachment

Each e-mail in TNEF has a priority field:

  • Low – e-mail with low priority
  • Medium – e-mail with medium priority
  • High – email with high priority

Latest specification of TNEF format is available at [MS-OXTNEF]: Transport Neutral Encapsulation Format (TNEF) Data Algorithm page.

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