One of the most popular ways to fight spam is certainly to rely on the Spamhaus Project. Founded in 1998 by Steve Linford, Spamhaus provides a series of DNS Block Lists that collect hosts and IPs that are considered sources of spam.
To put it simply, they have created a huge blacklist open to any mail server: if a server implements Spamhaus, it will automatically reject any messages coming from a blocked IP, reducing effectively the incoming spam rate.
The organization also provides a useful Spamhaus Lookup to check whether your domain or IP has been blacklisted (and, in case, to ask to be removed):
There are three main lists, the most important being SBL – Spamhaus Block List – that is, "a realtime database of IP addresses of spam-sources, including known spammers, spam gangs, spam operations and spam support services", updated daily. The other two, XBL (Exploits Block List) and PBL (Policy Block List), collect spammers using botnet and proxies.
To get rid of all these problems, however, turboSMTP provides a professional outgoing server that relies only on certified IPs: making your mailings always secure and avoiding you to end up into junk folders.