Some email servers and spam filters reference "blacklists," which are lists of IP addresses of known spammers, or "spam friendly" servers. If an IP address is on the list, they won't let your email through.
Anyone can setup a blacklist for their own email server, and share the list for the public to use. Not all blacklists are created equal. Smaller blacklists, including SORBS, DRMX, Lashback, NoSolicitado.org, and Spam Cannibal have less influence with the bigger ISPs. Most large ISPs only use the most reputable blacklists like Spamhaus.
If a server gets on a blacklist such as SpamHaus, any email from the server is blocked by ISPs that use the SpamHaus blacklist. SpamHaus and other reputable blacklists usually have a threshold, and usually delist servers after a reasonable period of time.
Sometimes, blacklists and spam filters don't just ban a specific server's IP address—they ban an entire block or "range" of IP addresses. This basically blocks all email from all the servers setup on that general IP range (for instance, 69.20.10.???). This is a drastic measure, but spam is a drastic problem. If your IP address is on a blacklist, it could be because someone in your IP range is a spammer.
Find out if your IP address is on a blacklist at http://multirbl.valli.org/lookup/.