By the way, Luke, do you mind me asking a question in return ?
Since you have custom domains with dyndns.org, do you know if they provide Backup MX services (ie. have their servers act as backup MX servers to our own) ? If so, does it cost any extra above the fee for the custom dns ?
I know there is a backup MX check box with the dynamic and static IP services, but I cannot activate them, so I don't know what they are for.
Thanks.
Kelvin