Hi Julius,
This functionality, i.e. Alert Mails for only Red Alerts are available from SolMan 7.1 SP9, not before than that.
For lowere versions, if you want to enable this capability, you need to implement a BADi, but have not heard of any success stories around this BADi approach, so far.
So, please wait for SP9, as I am 
Thanks,
Hanfi