If you firewall out SMTP into the network except for the gateway then that should help you avoid doing a Demon and getting your outgoing mail relay listed in an RBL even though it's actually a customer's fault.
What's an RBL? I use Demon as an ISP, and with a paid e-mail account I was only able to use demon's SMTP server, rather than the one I paid for... is this due to relaying? Would this also affect me if I had an MTA on my box at home?
Ricardo