someone trying to access my email server

I have been checking the logs to my email server and am noticing that someone is trying to access my email server for whatever reason. The error message that I am getting in the application log is

“Logon attempt from xxx.xxx.xxx.xxx to “random name” failed. HrLookupCredentials() call failed with error Logon failure: unknown user name or bad password.

What is the best way to prevent this from occurring? We are running Exchange 5.5 sp4 + all updates following sp4 on NT server sp6a.