Is There Any AntiSpam That Verifies Use ID?

I need an anti-spam service for my company that will only allow pre-authorized users to send me emails and require unkown users to a challenge answer verification.

There is spamarrest.com but I need to compare with many many others. Could you please list me some USER VERIFICATION antispam services.

Thanks.

Is There Any AntiSpam That Verifies Use ID?adware remover

Are you sure you want to use this? It seems like it would irritate customers who are emailing you. If I saw a bounce back message like this, I probably wouldn't click it. Also, you lose your domain name and have to use theirs for email. And their process of logging onto webmail to check for unverified email sounds the same as logging into webmail and checking your junk email folder. In others words, just as complicated. Also you might want to check if you can globally control settings for your whole company or are stuck doing it ala carte. It might make things easier for your employees but more of a hassle for your customers.