This permission should probably not be mapped to `add`.
The difference between `ftw.mail: Add Mail` and `ftw.mail: Add Inbound Mail` is that `Add Mail` tells that I see "Mail" in the add-menu, but `Add Inbound Mail` gives me the permission to create a `Mail` without uploading it.

I think a normal user should in general not be able to create a mail manually, but he should be able to do this with mail-in.

Therefore I propose mapping `ftw.mail: Add Mail` to the action group `manage`.