I could be wrong, but seems to me that once you drag a "mail item" to ANY folder that is in the "on my Mac" section, an actual copy of the entire email (not just the header, with the actual content of the message remaining on the server) is created on your Mac, and, the copy that was previously "on the server" is then deleted.
So, if the emails were ALREADY IN a "trash" sub folder located in your "on my Mac" area, if you execute an "empty trash" command, they might be gone for good, and ARE NOT "recoverable".
Again, I could be wrong. Others, please correct me.
A followup...
I created a folder named "Trash" in "on my Mac".
I put some emails into it.
I then used the Menu command Mailbox -> Erase Deleted Items -> In All Accounts.
After doing this, I checked the "Trash" folder in "on my Mac", and the items WERE STILL THERE.
HOWEVER...
If you do the following:
- Open the "on my Mac -> Trash" folder
- Select one or more items in it and delete them (or otherwise move them to the trash)
- And NOW choose the "delete mail" in all accounts command...
then...
The items now appear to be "gone" and not recoverable.
So...
Whether or not the emails in question are recoverable... depends on what you did with them...