Bluebottle - Free Anti Spam Email
Free spam-protected email with 250MB of storage, POP3 and web-based access and outgoing SMTP. Bluebottle only accepts emails from trusted senders making your email spam-free.
http://www.bluebottle.com

FastPCNet : Spam and Virus Filter
Offers redundant spam and virus filtering service with spam filters located in carrier neutral data centers in different regions of the country.
http://www.fastpcnet.net/spam-blocker.html

GMail
Google operated web mail service based upon search functionality.
http://gmail.com

Mailtrust Business Email Hosting
A division of Rackspace, specializing in business-grade email hosting since 1999. Provides corporate email hosting, email marketing services, and reseller accounts to small and large businesses.
http://www.mailtrust.com