I have just reproduced your scenario - I sent a mail to one of my other identities, retrieved it from the Sent mail folder, positioned the cursor by a word and hit the Delete key - whereupon the mail disappeared.
It has turned up in my Mail Trash folder (not the main Trash folder) - is this what you meant to say? If it's not in your Mail Trash, then I don't know where it is. Forgive this (maybe) stupid question, but you didn't empty the Mail Trash folder before realising your mistake? Or do you have the Mail Trash on an auto-empty schedule?