1. Different reasons for doing special routing

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.