Imagine yourself with pen 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
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 UUCP and BITNET). This would be like the post office using
United Parcel Service to deliver an overnight letter.
This analogy between a post office and sendmail
will break down as we explore sendmail in more
detail. But the analogy serves a role in this introductory material,
so we will continue to use it to illuminate a few of
sendmail's more obscure points.