Recipients receive a winmail.dat attachment

PROBLEM

For some emails, the recipient receives a text message with a winmail.dat attachment, no HTML part.

SOLUTION

Some emails, very few recently, are sent in an old legacy and proprietary TNEF format which is not supported by all clients. Moreover, security checks and html disarming may interfere with such format.

One work-around is to disable such format on the sending side. Here is a Microsoft document that explains how to do it:
https://support.office.com/en-us/article/Recipients-receive-a-winmail-dat-attachment-1735BA97-39B8-40D4-BA17-0E0150EF87A8

If the above work-around can’t be applied, it is possible to change the way Libraesva ESG handles TNEF:

1. “no replace“: Libraesva ESG opens the TNEF file and analyzes the content (message and attachments), without converting the message into a standard MIME/HTML email.
2. “replace“: Libraesva ESG opens the TNEF file and analyzes the content (message and attachments), converts the message into a standard MIME/HTML email, doing URLSand link rewrite and HTML content disarm as with all other messages
3. “auto“. Libraesva ESG deals automatically with TNEF switching from mode 1 to 2 based on the analysis done. This is the Libraesva ESG default setting.

There’s no perfect solution managing TNEF, and all the above have to be considered as best efforts. Any of the above solutions have advantages and problems, due to the TNEF proprietary format that works without problems only if the whole message chain is using Microsoft products. Reading the above options it seems that option 2 is the best as Libraesva ESG can apply all of its security features, delivering a standard MIME/HTML message; unfortunately this includes calendar invitations, that no longer work with a Microsoft client for example.

We strongly recommend to leave the default option “auto”, but if for any reason you need to change this setting, proceed as follows:

• Open the hidden advanced page setting on your Libraesva ESG athttps://libraesva ESG IP/libra_esva_advanced_settings.php
• Select the “MS Config” Tab
• Search the option with external name = “Use TNEF Contents” (search usetnefcontents in version 4.x)
• Change the value and set it according to these possible ones:
  • Option 1 above (no replace) -> “no“
  • Option 2 above (replace) -> “replace“
  • Option 3 above (auto) -> “add“
• Click on Restart MailScanner button on the toolbar