I have to frequently stay on top of spammers, both to my mail server, and blogs. Since I host my blogs, the mail server, the web server and the database server, I have to ensure that each of the roles are functioning properly at all times.
Should the mail server break down, it's not an issue, since other mail servers keeps retrying to send mail for the next few hours (or, for some, days) before it gives up.
Should any of the other server roles fail, then the blogs and the web sites immediately become unavailable. Fortunately, such occurances are rare. I keep a daily backup of all the databases used to host the contents of the blogs and sites.
Keeping spam out of the mail server, however takes some more effort. This is because the appearance of spam is normally precipitated by the simple connection and disconnection of hosts. Once these symptoms appear, those hosts are blocked from connecting to the mail server in the future. In addition, known spammers are blocked by domain names, IP blocks, regular (or wildcard) expressions which, when expanded, conforms to the blocked domain names.