Well, you need (of'course) a Linux machine, what can be connected to the internet - So! (i am smart - i am smart :)) - you probably got a modem, right? a phoneline - or any way to connect yourself with the server you are running. You will also need some software like Fetchmail, Procmail, an extra account, and your email account or DNS with email account.
This can be ANYthing ... even a 8086 ... - but preferable - since there will be some load on it - minimally a 80386 :). as faster the machine - as faster the software will run (rite eh)? :).
You need to create a extra account on your Linux machine. I am using as example (in this HOWTO) "mailservice". This can be as well "mailserver" or "mailtousers" - as long it is some name what can be recognized by you!.
! It doesn't need to have ANY root privileges !
This mailaccount will forward all mail to the users on your system, or to external users (not on your system :)). Just create it as an ordinary user. You will need to test things under this account, and, the mail "administrator" can use this account to administrate the mail account without being root even .. so - pretty safe !.
I am using v1.9 patch level 9 ... i found it at sunsite.unc.edu, and, i am using only this mail-fetcher in my example - if you want to use another one, you are on your own ! since i am happy with it :). For extended features you should read the man of Fetchmail :). This can be installed as root - as well for the user itself ... - best is to install it as root :) since i know it will work for sure then :)))
I am using v3.10 - found it again at sunsite.unc.edu, and - i like this as the best mda (mail delivery agent) around ... It delivers nicely - and it is better than using it by the "original" system mda - since it will deliver user-per-user ! ... The same as above - it can be installed as root, or as user - and :) best is to install it as root - since i know it will work for sure then (again!) :)... how repetetive ...
You always need to have a mail account - where you can "poll" your mail from. The server needs to be a POP server - where you can poll from when you want, and where users can send their mail 24/24 to. It can be a free-mail service, as well commercial - even your mailaccount is sufficient enough. In the "subject" method (m(B)) - you don't need something more - in method A - with the "to" fields - you NEED more - as written below !
Needed to run your own "domain" - email domain ... it is needed since the administrator of the server can't use it's own domain for your users - since there are users who need to be contacted on his server as well - what could interfere with your users. And your mailheader must be filtered on that domain. So - an apart domain - pointing to HIS server. This can be an additional cost - or you can ask your system administrator to add a "non- authorative address" - pointing to his server ... - however - it needs to be another domain than he is using !
Well, the system administrator needs to forward *ALL* mail to your domain to YOUR account ... - it is easy for him when he knows how to do it :) .. else he will need to read the sendmail manual :)) (or qmail - whatever). You will poll for your mail at your account and voila ! filtering on YOUR own email domain.