NeverBounce - Real-time email verification and cleaning to ensure emails never bounce.
Mailwasher - Over 8 million people use MailWasher as their spam filter
Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI
SPAMfighter - SPAMfighter and Microsoft have built the strongest and most effective anti spam filter for Outlook, Windows Live Mail, Outlook Express & Thunderbird
ZeroBounce - Removes invalid emails from your list to prevent email bounces from ruining your deliverability.
Spam Reader - Spam Reader is a free anti-spam add-on for Microsoft Outlook.