You could also grab it with a CGI program, and there are some out there that do this. HOWEVER -- some folks travel with their email info munged (I do also) and not every visitor is a real prospect. And some of those are active spamcoppers (me, for one.) Others may be angry at the "invasion of privacy" if they just snooped by and found it of no interest and left.

Safer bet is to give them the option of leaving their email.