Prerequisite:

You have routed your internal email traffic through our systems and have configured signatures in the Signature and Disclaimer module.

Manual entry for configuring email routing: Routing internal mail through Hornetsecurity

Manual entry for configuring Signature and Disclaimer: Signature and Disclaimer

Cause:

Microsoft Exchange and Outlook clients can send emails in a proprietary message format called TNEF, which prevents adding signatures and disclaimers through encapsulation by Microsoft.

The Transport Neutral Encapsulation Format (TNEF) is a Microsoft format for encapsulating MAPI messages. It contains a plain-text version and an attachment named “Winmail.dat,” which holds the formatted original message. You can also quickly recognize in Email Live Tracking whether a message was sent in this named format by this attachment.

Solution:

Option 1: Disable TNEF via PowerShell

Set-RemoteDomain -Identity <domain.tld> -TNEFEnabled $false

Option 2: Disable TNEF via the Microsoft 365 Admin Center

1. Log in to the Microsoft 365 Admin Center.
2. Open the Email flow section.
3. Select Remote domains.
4. Open the remote domain Default or the desired remote domain.
5. Under Use Rich Text Format, open the option Edit text and character set.
6. Set the setting to Never for Rich Text Format.
7. Save the changes.

If the problem persists, please check a specific example and gather all relevant information. Our support team will be happy to help you find a solution and resolve the issue.