Imagine yourself
withpen and paper, writing a letter to a friend far away. You finish the letter
and sign it, reflect on what you’ve written, then tuck the letter into an
envelope. You put your friend’s address on the front, your return address in
the lefthand corner, and a stamp in the righthand corner, and the letter is
ready for mailing. Electronic mail (email for short) is prepared in much the
same way, but a computer is used instead of pen and paper.
The post office
transports real letters in real envelopes, whereas sendmail transports electronic
letters in electronic envelopes. If your friend (the recipient) is in the same neighborhood
(on the same machine), only a single post office (sendmail running locally)
is involved. If your friend is in a distant location, the mail message will be forwarded
from the local post office (sendmail running locally) to a distant one (sendmail
running remotely) for delivery. Although sendmail is similar to a post
office in many ways, it is superior in others:
·
Delivery
typically takes seconds rather than days.
·
Address
changes (forwarding) take effect immediately, and mail can be forwarded anywhere
in the world.
·
Host
addresses are looked up dynamically. Therefore, machines can be moved or
renamed and email delivery will still succeed.
·
Mail
can be delivered through programs that access other networks (such as Unix to
Unix Communication Protocol [UUCP] and Bitnet). This would be like the post
office using United Parcel Service to deliver an overnight letter.
