Ask HN: Obligation to pay recruiter for semi-unsolicited referral?

If a recruiter sends an email to you offering a candidate and asking if you want to see their resume and get contact info and you say 'yes' and then hire the person, but no prior arrangement has been made with the recruiter, is there any ethical or legal obligation to pay the recruiter? This seems to happen a lot, so why do recruiters do this? Note, the email header says "This resume submission has been solicited by your company. Any contact with the candidate is to be initiated through [recruiting agency]" but the submission was NOT solicited, at least not from them, and no contract or discussion of payment was made. We've made an informal offer. How should we proceed?