Are you referring the delete key on an external eg: B/T keyboard with the eM Client iOS or Android app ?
Or are you referring to the desktop client del key ?
The del key for emails on the desktop client already works. If yours doesn’t, go to “Menu / Settings (Preferences) / General / Shortcuts” and check the Del key is assigned.
If you are using an external B/T keyboard with the iOS or Android app, the delete key “is not supported as yet” as well as the arrow keys & some other external keyboard keys as @Michal_Burger advised in the following thread.