Recommendations to prevent spam attacks

Recently one of my clients, had a virus and stole the mail access passwords, my server when crowd started getting spam mail sent through the correct login and password my client, until I found out and I act passed a few hours, when I went to the server and started working, I saw that all emails were from the same user, I changed the password and even got to remove it user but then I saw they had thousands and thousands of emails in the queue, which are impossible to valid and which are not as fast for you to regain the service was erasing them all, which for a few hours some of my customers lost emails.

What options are to avoid these problems? Things that Exchange can do ...

corrective: There is an option to prevent sending more than "x" emails per user per day? etc. ..

preventive:

Notices to increase the mail queue, for example more than "x" mail queued, warning email to admin .. etc. ..

You can do this with Virtualmin???

I would like in this post read your answers or ideas to avoid these problems, unfortunately so common lately.