a lot of spam nowadays is sent with the explicit purpose of pinging email names and bypassing filters, just to find out of the address itself has a person that checks it. war-emailing, as it were

perhaps that email had something like an embedded image or javascript that could/would dial out to tell the spammer that your email address is active.

other tactics i've seen are the "unsubscribe to this email" links on blatant spam which are social engineering attacks to trick the unwitting into telling the spammers that yes, someone is home at this address.