How to protect against compromised email account password

[Admin Beckspaced]

Dear Postfix Users,

just recently the computer of a client got infected with malware and the 
email password was compromised.
The bad guys immediately started sending out spam emails via our mail 
servers.

We got notified by our monitoring system a bit later ... and fixed things

But lots and lots of spam emails have been sent via out mail server.

How do you protect your mail system against a compromised password and 
mass spam mail sending?

Thanks & greetings
Becki