Straight from the user guide:
IMPORTANT
Even with POP and IMAP configured for public access, users outside your local network are not able to send e-mail using your server as their SMTP host. Allowing this would open your server to abuse by spammers as a mail relay. Users who are travelling should either: a) use the STMP server of their local ISP; b) use PPTP to connect to your internal network; or c) use webmail to read their mail. Webmail provides your users with secure access to both read and send mail via your server.