NeverBounce - Real-time email verification and cleaning to ensure emails never bounce.
Spamihilator - The free Anti-Spam-Filter Spamihilator removes more than 98% of spam mails before they even get into your inbox.
Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI
SpamAssassin - Use this program to "Unspam" your mailbox(es).
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.