Dealing with sendmail errors

I noticed last week that my emails were not being delivered properly. After taking a look at when the issue started, I realized that it was about the same time I updated my box.

My email was being returned with the reason: 550 Banned rcpt apache@localhost.localdomain. There was also another error 554 5.0.0 Service unavailable. Both of these errors on their own were not to helpful. I asked a friend (the one who sold me the box) to help and he gave me some good advice, but I was unable to resolve the issue and several Google searches didn’t reveal any solution.