Your LAN client mailers can generate outgoing Internet activity when they submit messages via SMTP, and this can force your dial-up link to go up every time they send a message.
To avoid this problem:
- make sure that your own CommuniGate Pro Server, not the ISP mail server is specified in the client mailer settings as the "outgoing mail server";
- make sure that IP addresses of all your LAN Clients are included into the Client Hosts list;
- make sure that SMTP module Verify Return-Path for option is set to non-clients or nobody.