You're not kidding aboutt the local telephone rates. They are a ferociously high!
Is the SMTP Server local or at your ISP? If it is not on your local network the behaviour you are describing would be correct. However, to restrict it you have two options. One, you can use the local-sites option in the Dialer page. That allows you to specify sites which the dialer will not establish a connection for. The other option is to select the specific dialer profile and to start using the Access Right Policies there.
Then you can specify which ports (110 and 80, for example) and which types of requests are allowed to trigger the dialer by using the Advanced Filters and Criterion. You'll find a rough overview of them
here. The same principles apply.