Kevin,
Nice to hear you are getting into Method now. This is a great question by the way. To give this answer proper justice, I need to give a little background information on how mail servers work.
For this example, I am going to use mail.taylorlawn.com as the example. "mail.taylorlawn.com" can have several users under its domain, regardless of the type of mail server it is (Exchange, QMail, etc.). Most servers now use authentication. If you aren't using authentication, you should be otherwise your server could turn into a relay. A relay allows other people to use your server to send out SPAM. Not good.
When you go to send an email from your server, the first thing the server checks and cares about is if you, the user trying to login, has access to this server. If not, you are booted out. If you are authenticated, you get the green light. Once you have the green light, the server doesn't care who you put as the "From" and "To" in the email. It simply sends what you tell it to send and does so because you (the signed in user) has been authenticated and is allowed to do so. When a person receiving the email replies back, it simply looks at the "From" address and sends the reply back to that email address.
Therefore, if you put in your login information for the “mail.taylorlawn.com” server but put the “From” address as someone else, the sender (or the assigned to) will get the reply.
Hope that helps!