Many servers on the internet will reject all traffic coming from certain IP's if they are on blacklists.

The following websites will assist in checking on all aspects of a domain, i.e. dns records, reputation, blacklisting etc.

If you are unfortunately blacklisted then use these sites to resolve the issues.

DNS Reporting

http://www.dnsstuff.com/ 

MX Toolbox - Blacklist Checking

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

Senderbase - Domain/ IP Reputation Assessment

http://www.senderbase.org/

TrustedSource - Reputation Assessment

http://www.trustedsource.org/

DCC - Distributed Checksum  Clearinghouse

http://www.rhyolite.com/anti-spam/dcc/reputations.html