OT: how do You protect an email relay service?

[Sthu Deus <sthu.deus@gmail.com>]

Good day.


If You use an email relay service, how do You protect it: VMs, iptables
connections rate limit, ... ?

Personally, I have a problem with email sending authorization - how I can
separate the users that have not their boxes on our service and therefore I can
ban their trials to pick up a password - I can not reduce it even to the local
net IPs bt iptables - as port 25 is used for not only for sending our own users
but for receiving it for the local users - as I understand.


Thank You for Your time.