ACT! as e-mail client

Does anyone know a good source for information about using ACT! as an e-mail client? I am trying to determine if I can use ACT! for a customer who has a domain, but uses an ISP to dial-up. I need different usernames and passwords for the POP and SMTP servers. I’ve tried their technical support suggestions and can get ACT! to send and receive mail, however the recepient sees the ISP’s e-mail as the reply address (me@myisp.net). I need the recepient to see the domain name as the reply address (me@mydomain.com). Other e-mail clients allow a “My server requires authentication” option.