Try to find out which blacklist you are on, and work with its maintainer to fix the problem. These links might help:

http://senderbase.org/senderbase_queries/rep_lookup

http://rfc-ignorant.org/

http://www.mxtoolbox.com/blacklists.aspx

http://multirbl.valli.org/

Also make sure your email list is double-opt-in so that malicious people cannot sign up other people for email.