Guys,

Need advice of those who have programming understanding. I have an idea of launching a free service that filters people's e-mail accounts from spam based on most popular filters.

What am I looking at from hosting-needs points of view? What will I need technically to implement? How much could it cost? How much bandwidth could it consume?

Thank you.

Would you provide email hosting to these people? Otherwise, you would need to come up with plug-ins for their mail clients or develop your own mail client as well. Or maybe you can setup a Perl script that connects to external pop servers and removes spam mail.
As for the filtering itself, any program language will do, so any host with PHP or Perl support would be fine.

John, that's a lot of help. That's exactly what I pla to do - just a plug-in that blocks spam, not a hosting itself (I'll leave this to you guys). How about the potential bandwidth consumption? Starting with shared - will this work?

Thank you.

If it's on the client-side (a plugin for Outlook Express for example), then you don't really have to worry about bandwidth consumption would you?