In case you think that most of your clients use Outlook, let me tell you from antecodal research that webmail are some of the most viewed pages. Hotmail and Yahoo mail are both webmail. Now Verizon had to revamp its webmail interface to AJAX (see telephony story). Why?
Verizon needed a next-generation Web e-mail client to keep its broadband customers using its portal applications and content. E-mail drives the majority of customer page-views on the portal and serves as a foundation upon which Verizon can up-sell and cross-sell other content and services.
What are you waiting for? I woul dhave webmail redirect at logoff to a portal that looks like CraigsList. And I would let my paying customers post for free; but others pay for an ad. Besides classifieds, I would add blogging, RSS, video+photo sharing. But Stickiness is hard work