I just discovered something curious. I accidentally pasted when I meant to undo in Apple Mail, and it took the text on the clipboard and created a new mail message with the clipboard text as its contents. Not a new message ready to send, like if I had dragged the text to Mail's icon, but a new unread message in my mailbox that had the text as its body and no headers.
"That's strange," I thought, so I pasted again, into TextEdit, to see if I'd somehow copied a mail message or something - no, just text. I then added a Subject: and From: header line, copied and pasted again. The same thing, only this time with those headers. It only seems to work with plain text pasteboard contents, and I couldn't get it to work reliably in every kind of mail folder, but a local mailbox seems to work most consistently.
I feel like I've found Mail's vestigial tail - this is mostly harmless behavior, but a little confusing, and I can't really think of any use for it. I'm curious how it got in there.