There are many technical reasons why one would want to take
control over sendmail's normal behavior. Here are some:
- The destination host is not reachable from your computer.
Your computer is behind a firewall.
- The DNS of the destination host is buggy or incomplete
(no valid MX record).
- The host is reachable by UUCP only. You get this situation
when one has a valid domain, but no direct connection
to the Internet. Your computer might be the designated
Mail eXchanger for this domain. As such, it will receive
all email for this domain. It will need special rules
to forward the messages properly.
- You have a better (less expansive, faster, etc.) route
to this host than the default one.