I would imagine that this is because you are not dialling into our network. Maybe you have GPRS?
If this is the case then you will not be able to 'relay' mail through any SMTP server, other than the one which is configured for the network you are using.
If you contact your network / phone dialup provider then they should be able to advise on the correct setting for your outbound mail server. By changing this, you will still be able to collect mail normally from our POP3 server - SMTP is an unauthenticated protocol and has to be locked down in this manner to prevent abuse from spammers.