Quite.
You might have mail.freecheese.com, mail.tonsilhockey.com and mail.kermodebear.com, but if they are all on one server with one IP, your mail is all going to come out from one of those (or a 4th "placeholder" like mail.corp.com) despite being sent "from"
x@freecheese.com,
y@tonsilhockey.com or
z@kermodebear.com by the user.