Spammers try to defeat spam filters by making their emails look as if they've come from a legitimate source. To do so, they enter a valid email address (possibly obtained by hacking into a legitimate website) in the 'From' and/or 'Reply' field(s) of their emails. That then results in the person who owns that address receiving lots of complaints and, possibly, finding that his/her own genuine outgoing messages then get blocked by spam filters.