As mentioned earlier, Postfix typically attempts to deliver messages as soon as it receives them, and again at regular intervals after that. If you have only periodic connections to the Internet, as with a dial-up connection, you likely would prefer Sendmail to hold all messages in the queue and attempt to deliver them whenever you connect to your ISP.
As dialup connections have become the exception rather than the rule and are now quite rare, Ubuntu does not configure them by default and does not include the
pppd daemon in the default installation. If you need this, install
pppd from the Ubuntu software repositories. You can then configure Postfix to hold messages for later delivery by adding the following ...