Message Routing - Security Enhancements for TNEF Encapsulated Items - May 2024

Service Update

Availability	May 6th, 2024
Product(s)	Email Security Cloud Gateway (CG)
Who's affected	Email Security Cloud Gateway (CG), Administrators

Overview

Mimecast is pleased to announce the general availability of enhanced security for TNEF-encapsulated items.

What's changing

Transport Neutral Encapsulation Format (TNEF) is a proprietary email attachment format used by Microsoft Outlook and Microsoft Exchange Server for encapsulating message properties. All versions of Microsoft Outlook support TNEF.
Other email services display items that include TNEF-encapsulated content as plain text messages with winmail.dat attachments.

To learn more about TNEF, please review the following Microsoft articles:

• TNEF and Internet email messages.
• TNEF conversion options.

Although Mimecast, from a visibility point of view, doesn’t support TNEF-encapsulated messages, such items are supported by our Security features.

Via this Service Update, we are announcing that we will enhance our Security capabilities for TNEF-encapsulated messages so we can examine them more thoroughly. Especially when messages have nested TNEF-encapsulated items.

Recommended actions 

To stop emails from being sent with Winmail.dat attachments, Mimecast recommends that customers review the Microsoft documentation (linked above) and consider disabling TNEF for remote domains.

After the release, administrators might notice an increase in Detections, due to the increased efficacy of the Mimecast Security features.