MXtoolbox has a nice resource to check a mail server IP address against 147 DNS based email blacklists. (Commonly called Realtime blacklist, DNSBL or RBL). If your mail server has been blacklisted, some email you send may not be delivered. Email blacklists are a common way of reducing spam. If you don't know your mail server's address, start with a MX Lookup.

Check to see if you're on a blacklist.