Does anyone know if there is a master list anywhere of current ISP email addresses?
What do you mean exactly? ISP-provided email domains? (For example, sbcglobal.net, att.net?)
Try working off the "Inferred Company" system field and sourcing a list of ISP company names?
You can refer to the list provided here: List of Internet Providers . This has ISPs for all the regions. However, if you have to manually pull out the domain for each ISP or use some data extraction / scraping app.