It's probably gone for good. Here's some info on recovering deleted emails, but not drafts.http://docs.info.apple.com/article.html?artnum=61449
Suggestion - For lengthly & important emails, prepare the draft using Text Edit & save after each update. When complete, copy & paste into Mail.
I just did this as well -- that is:
"I was just looking at the mail, not intending to edit. So, I didn't open the draft mail. Unfortunately, I spotted a mistake and went to correct it....but selecting text and hitting delete will delete the draft email instead of deleting text in the email when you haven't opened the email in a separate window."
In MacMail it would have just gone to the Trash floder in mail -- I, unfortuantely, am on iCloud on my PC at the office looking at a draft I typed up this morning on the Mac at home. It is NOT in the trash folder and it's NOT hiding under another active window. It's just gone...
Where do deleted drafts go in the iCloud world?!!
(The trash folder would make sense but that ain't it...)