682Part IVConnectionsThe major parts of our model e-mail (Web server type)
682Part IVConnectionsThe major parts of our model e-mail system are as follows: .TCP/IP server .Mail Transfer Agent (MTA, aka SMTP server) .Mail spool .Mail User Agent (MUA, aka mail client) .Mail retrieval program, aka POP/IMAP server .Mailing list manager (MLM) Let s examine these in greater detail. We re going to use a cheesy but apt metaphor to helpyou remember who does what: Mail Server Mansion. TCP/IP serverA good way to visualize a TCP/IP serveris to think of it as the footman who answers everyknock at the front door of Mail Server Mansion. Actually, if you think of every separate portas a door, maybe the TCP/IP server is more like a security guard who sits in a cubbyhole andmonitors a bank of video cameras pointed at multiple avenues of ingress and egress. In anyevent, it s simpler and more picturesque just to think of an old-fashioned single footman at asingle door. The footman does not actually speak to any visitors. He simply opens the door when he hearsa knock, recognizes the type of interaction this will be, and calls the correct person (say abutler or a bouncer) to handle the request. This second person, who has a speaking role, willtake over to find out what the visitor wants. The point is not to open the door it is to know there is a door to be opened. A TCP/IP servermaintains a list of several services for which it is responsible, and it answers each request byinvoking the proper daemon in the case of mail, either sendmailor a POP/IMAP authenti- cator. This saves resources because a single daemon monitors all the requests and the otherdaemons are only invoked on an as needed basis. Well-known TCP/IP servers include: .GNU inetd(the TCP/IP super server usually found by default on Unix setups(www.gnu.org/software/inetutils/inetutils.html) .xinetd(a secure replacement for inetd (www.xinetd.org) .Dan Bernstein s tcpserver(often used with qmail (http://cr.yp.to/ ucspi-tcp.html) Mail Transfer Agent, aka SMTP serverThe Mail Transfer Agent (MTA)is the heart of any mail server, and the part whose workingswe are most radically simplifying in this discussion. The most important task of an MTA is to accept e-mail from another SMTP server and deliverit to the correct addressee s mail spool. A lot more is involved in this than one might think, but we can t get into it here. Also, the MTA will collect outgoing e-mail and try to send it toother SMTP servers.
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