Limiting Number of Messages a user can send per hour Qmail / Plesk

[favs33]

Hello,

We are using Plesk / Qmail. Does anyone know how we can limit a user
to only send X number of messages per hour?

I have asked Parallels support to no avail. The Qmail website also
does not show any patch that does this. Certainly someone must have
done this before, right?

What we are trying to accomplish is the same idea as described about
these ISPs:
http://www.emailaddressmanager.com/tips/server-smtp-limit.html

Thanks!