Trident for Windows offers two spell checker options: the Native Windows spell checker and the Zia spell checker.

To enable the Zia spell checker:

1. Click the Main Menu icon in the top-left corner of the app.

2. Select Settings from the drop-down menu.

3. In the Settings window, go to Mail and click Mail Composer.

4. Toggle the Spell Check option On to enable it.
   

Notes:

1. The native spell checker is enabled by default in the system.
   
2. This setting applies to all emails you write. If you turn it off once, it will stay off for every new email until you turn it back on.