The questions you where asking are seemingly unrelated to the task of maintaining or developing a "mail service".
While it is good and even encouraged to set yourself goals, you really need to understand your limits. This is not stuff your even going to learn overnight let alone become experienced enough to run a trusted service with. Have you any experience maintaining unix servers? Have you any experience with a proper firewall such as iptables? Have you any experience installing and configuring properly an smtp server such as postfix or exim? Do you know what I am talking about?
People who do not understand a problems scale will often mistakingly assume it as "simple" or as "easy enough to learn". These people quickly forget that more complex problems are often solved by people with allot of experience in the field.
Don't get me wrong. Given enough time and ambition anyone can achieve there goals. However, coming to a community of programmers, server administrators and IT professionals who can see instantly that you are a beginner then writing off there advice as discouragement for the sake of being discouraging. Seriously, you need a wake up call.