Decrypting PDF Messenger Email from PGP Encryption Server results in winmail.dat (and is unreadable with TNEF Encoding)
Article ID: 385099
Updated On: 03-28-2025
Products
Issue/Introduction
PGP Encryption Server has the ability to send encrypted/secured content to recipients who do not have encryption software to access it.
One such feature is PDF Messenger in which the message is encrypted to a secured PDF. As part of this, the attachments and text are encapsulated into a single secure "PGPMessage.pdf" file.
When the recipient opens this file, they enter their pre-established password to decrypt and the entire message is then readable, just as the sender composed the initial content.
If the sender used "TNEF" encoding, which is an old Microsoft encoding used only for internal Outlook users for Outlook-only compatibility, the decrypted message will result in a "winmail.dat" that cannot be viewed without additional software or tools.
This article will discuss what the issue is and how to prevent it.
Environment
PGP Encryption Server with secure PDF Messenger.
Sender is using Microsoft's TNEF Encoding, which is a proprietary encoding for Outlook.
Resolution
If you end up with a winmail.dat file, you can be confident the sender has TNEF encoding. As mentioned, this is a proprietary encoding used by Outlook and no other program can read it.
This means if you end up with this .dat file, you will need to have the sender re-send it without the encoding enabled to properly view.
If the PDF Messenger message was decrypted, and you see "winmail.dat" to the right pane, which is where attachments go, then you may not be able to view it.
In order to avoid the "winmail.dat" scenario, you will want to ask the sender to disable "TNEF" encoding.
TNEF encoding was meant only for internal communications, and was not meant for internet-destined email, such as sending to an external recipient domain.
For example, if "user1@example.com" sends a message to a colleague, "user2@example.com", the Microsoft internal mailserver (Exchange) can parse TNEF encoding, and the result is the email looks normal on the recipient end. This all works because TNEF is understood by Outlook as a proprietary encoding.
Sending to the internet will break the message formatting and the result will possibly be, "winmail.dat". The "winmail.dat" is actually all the message content, encapsulated with the TNEF encoding and as a result, if Outlook is not the parser, the content cannot be viewed.
So if "user1@example.com" sends to an external user, "user2@example.net", the message formatting, and content could be modified and may look significantly different, or even unreadable to the external user.
in order to avoid this, the sender should ensure that TNEF encoding is disabled for all outbound email--that is, any emails that will go outside of their own internal domain, will need to disable TNEF Encoding.
The following are articles that will discuss how this can be done:
Topic 1: How message format affects email messages
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