Yes, I have this exact same problem. However, I do not keep other people's e-mail.
I have been able to track down the correct people to whom the e-mails belong. In two cases, the people are lawyers and the e-mails contained either personal or confidential information. Another case is a general contractor, and I've received quotes from subcontractors, blueprints and general correspondence. In one case it was a confirmation of tickets for a theme park. (I debated showing up as soon as the park opened and claiming the tickets, but ethics got the better of me.)
These people now reside in my address book. I forward the e-mail in question over to them, and CC a copy to the sender.
One guy kept signing up for things using MY e-mail address instead of his. (name@isp.com vs name@gmail.com) He finally got the hint when *I* got the login information for his match.com account. (Ethics was still distracted by the theme park tickets case and lost.) Considering he was a single lawyer in San Francisco, I think my updates indicating he was gay, submissive, into BDSM and wealthy might've paid off. He seems to be extra careful in which e-mail address he uses now.