When a guy emails me, I consider him to then be my boyfriend.

What I'll respond to the email is:

I am your girlfriend. Because you emailed me, I am now your girlfriend.

This could be any guy. It could be an Amazon customer service response guy. It could be a Brazilian saying his last name is also Peretti and he found me via a google search. Guess what? You're my boyfriend.

And it could certainly be a response to a casual acquaintance I don't know well enough to joke with in this manner.