sendmail, IMAP, POP3, proclaim, Outlook?

Hi,
I am newbe for mail server.
Want to clarify certain basics. Please correct me if I have understood wrong.
1) sendmail server is to relay incoming and outgoing email coming to port 25 and going from port 25 of server via DNS map.
2) DNS server should be installed on sendmail server machine.
3) IMAP server should be installed at server machine is to accept request from Email client such as Outlook express from client machine.
4) POP3 server I don't know. Is it replacement for IMAP?
5) Squirell mail is also Email client for Unix machines?
6) proclaim is for managing client emails boxes at server machine.
7) IMAP with SMTP AUTH(sasl authenication) can authenticate user who is accessing to sendmail server email ID from Client (Outlook Exoress) with either PLAIN login and password OR encrypted LOGIN and Password OR both as user requests.
Please help me to understand these basic issues. I am bit confussed with all this.
Thanks for support.

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