I agree that the action "Delete" is more common in these cases than the abbreviation "Move to the Deleted folder".
I was happy with either. That was until I thought about it. I think 'to deleted' is accurate and self explanatory. It just gets my vote because of the importance of the folders in an IMAP based mail system.
People used to Windows with a recycle bin might like 'delete', but I have at least one program on my PC that has a 'delete immediately' menu item, because 'delete' means put in a folder for real deletion later. Also in windows you can shift-delete or ctl-delete (whatever) to delete immediately as you have a physical keyboard.
On mobiles using iOS or android, I think the 'no entry' sign is used as a true irrevocable delete, but you don't use that.
In short, the term 'delete' is overloaded. I bet many users have apps on the same device they use with Aquamail for which 'delete' means 'delete', and no return ticket.
The more overloaded the term becomes, the more likely that a mistake will be made:
Either
Deleting irrevocably what you may wish to recover.
Or
Failing to delete irrevocably something that is still hanging about.
There needs ideally to be a certain and accident-resistant way to empty the deleted folder - in windows-speak to empty the trash.