If you are sending thru you ISP why are you trying to use a seperate domain? Most ISP have their own email domain and SMTP info. Using a seperate domain means you have to setup that domain's smtp.
What i mean is this. I have my email thru my ISP. email@example.com
at the same time i have my domains email. firstname.lastname@example.org
. While both are used for email i can not use the SMTP setting of my adelphia email to send mail thru my techmodo account. I had to setup my techmodo smtp in order to send mail that way.
The only way this would work is if you have a email server which uses your ISP's exact same SMTP info. Such as pop.gmail.com or smtp.gmail.com for GMail accounts. Other than that i dont see how it would work. Plus you also have to make sure that your Domain is setup thru your CPanel to send and recieve this way.
Unless i dont get it at all what you are trying to do and i am off in left field here.