Why should ISPs still pay to support subscriber e-mail either inhouse
or outsourced, any more than paying to support USENET, Chat, FTP/HTTP
Hosting, etc? Let subscribers choose whichever "free" or "fee-based"
supplier, and wash your hands of both the support issues and the legal
compliance issues.
For better or worse, whatever hoops you can make a customer have to jump
through to leave may keep them your customer 'by force'. Its hard to
change your email address and notify everyone on your address book and
the sites you may have used it to sign up with. It may not be right, but
it does seem to work.
Also, having your domain on that customers email address is low cost
advertising.
sam