[plug] e-mail route explanation pls - MDA, MTA, MUA, MSA(?)... ?

[Daniel]

Hi Plug,
OK I'm showing my ignorance rather than sit in silence: I don't understand 
the full route of e-mail going through a system.
Can anyone who has time help me a bit or point me in the right direction 
please? (I'm keen to set up a fully blown anti-spam script like those below 
- more detail if I ever get there).

My Hypothesis on How it works (for someone setting up a full system on a 
linux box to drag mail down from an isp and be able to filter it) is as 
follows:


Fetchmail goes and gets the mail [Not a mail delivery agent, but a 
remote-mail retrieval and forwarding utility - the best and the only one 
from what I can see)
Postfix [Mail Transfer Agent] (aka E-Mail Server?).  (I'm told that 
Procmail below could do it on its own, but an MTA gives better error checking.)
Procmail [local Mail Delivery Agent] can be made use of by Postfix to do 
local mail delivery.  Procmail does any filtering required - many scripts 
for fighting spam work are designed for Procmail.
Kmail [Mail User Agent] lets the user read the mail etc.

(have I got the right order that the mail would travel through from start 
to finish?)

and the solution I like most from the little I know:

Fetchmail
Courier IMAP (which comes with Maildrop and uses Maildir)
Maildrop (has standard .forward like Sendmail so mail can be passed through 
something like Active Spam Killer )
Evolution [MUA]

Thanks - any help in getting my brain around this is greatly appreciated.
Daniel.

PS
I have read things like
(in the unlikely event there is anyone out there who is floundering like me:
http://www.windowatch.com/2001/november/linuxmail7_10.html
http://www.linuxjournal.com/article.php?sid=1196&mode=thread&order=0)