Who’s It From?

In my pop e-mail account, I received a message [apparently] from a friend of mine. When I opened it and hit REPLY, the header of his message of course dropped down below where my reply text is to go. The strange thing is that I noticed the FROM: field contained his e-mail address (as expected) but the TO: field contained NOT my e-mail address, but rather his. This made me wonder whether he actually sent that mail or not. How would this be done to have appeared that way? If he typed in his own e-mail address, it would be sent back to him, not to me.

Puzzled???