iredmail: how to fortify security of relayhost

I have set up a simple linux mailserver using iredmail. But now I am totally worried that some hackers may find a way to hijack it for sending spam. What are the recommended steps to fortify the mailserver? (I haven't enabled any relaying yet, but I have icinga running on a separate server, and I am planning to use the iredmail server as a relay for icinga. Should I implement authentication?)