I don't think that they'd go as far as scanning mailboxes, the return on the effort doesn't seem worth it. More likely the person you were connected to added yo

I don't think that they'd go as far as scanning mailboxes, the return on the effort doesn't seem worth it. More likely the person you were connected to added you to an address book (or perhaps their mailer added you automatically) as someone they have dealt with before. Some anti-spam measures use a source address existing in a connected addressbook as a whitelisting clue.