email server concept: what's mail delivery agent

[Zhang Weiwu]

Hello. I'm trying to set up an email server several days. I think the 
most difficult part is to know when what does what. I wish to be clear 
on some concepts.

I am running cyrus + postfix + ldap on a gentoo server, multiple virtual 
domains (sealed mail server, no mail user have system account). It seems 
to me:

1. Moden mail servers don't often do mail forwarding. So upon receiving 
an email, it will always be 'local delievery'.;
2. Documents often mention 'delivery agent' and 'maildir', it seems 
'delivery agent' is not a standalone package, there is a delievery agent 
in postfix package, and there is also one in cyrus package, I can choose 
to use the delivery agent (postfix's or cyrus')
3. maildir is a way to store email, it is where postfix save incoming 
mail, and it is the place cyrus-imapd get emails so that to put to 
user's INBOX. But an email server can have no maildir completely, and 
let cyrus's delivery agent by pass maildir and put mail directly to 
user's INBOX.
4. putting the incoming emails in to each coorisponding domain's email 
storage place is the job of delivery agent.

I feel messed up reading all the documents, now I am afraid I got severl 
concepts completely wrong so I just write what I think plainly and hope 
someone kindly come out and say to me "hey, you got xxx wrong here", and 
I'll be thankful:)

Thank you.
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