To clarify my last response. Lets say you have a windows workstation with Eudora. It keeps its own outbox, inbox and folders.
ssmtp allows you do so much the same thing with a Unix workstation. You set up your mail client ... say ... xfmail ... to pull the mail from your pop3 account and to send via ssmtp. If you are offline, the mail will stay in xfmail's outbox.
