To open an EML file in Trident:
Right-click the EML file you want to open.
Click Open with in the menu list.
Click Trident from the list of available apps.
If you want Trident to be the default app while opening EML files:
Open Settings on your system.
Navigate to Apps > Default Apps.
Scroll down to Related Settings and select Choose defaults by file type.
In the search bar, type EML.
Click on the current default app next to .eml, and choose Trident from the list of available options.

Notes:
An EML file can be opened across all modules and services within Trident.
Actions such as Reply, Reply All, and Forward are available only when an EML file is opened from its local system location. These actions are not available when the file is opened within Trident as an attachment.