As an ISP we have a duty to prevent spam email on the internet. If a customer in our IP space starts sending spam we stand a chance of having parts of our IP range blacklisted which prevents us sending mail to that server. Mail servers receive mail on port 25 so we block port 25 outbound to prevent customers sending mail directly to the destination server. Instead we require an intermediary server to be used. For your average mail service (xtra, gmail, etc) the provider will host mail servers which customers can relay out through.
We are able to provide businesses who have limitations surrounding this, such having appliances, software that cannot handle doing SSL or allow a user to specify settings beyond a hostname. This is done via. SMX, a service that provides a sufficient quality of SPAM filtering that we can provide access to.
If you would like to inquire about SMX, please contact us on 0800 12 13 14 or emailing firstname.lastname@example.org