@paco To me, the main advantage of operating my own #email system is that I'm free to configure my inbound #MX so that it outright rejects "crap" (containing #malware or hitting a certain #spam score). With abusive mail, there's no way to reject anything once it was accepted, as this would mean #bounces, and therefore #backscatter.
BTW, you can allow your users to do a lot of interesting filtering stuff using #sieve (and manage-sieve). But probably not what you described here.