Mailwasher - Over 8 million people use MailWasher as their spam filter
Mimecast Gateway - Mimecast Gateway is a SaaS-based email security solution that uses sophisticated, multi-layered detection engines and intelligence to protect email data and employees from malware, spam, advanced threats and zero-day attacks.
SPAMfighter - SPAMfighter and Microsoft have built the strongest and most effective anti spam filter for Outlook, Windows Live Mail, Outlook Express & Thunderbird
dmarcian - Safeguard your email from abuse
Spam Reader - Spam Reader is a free anti-spam add-on for Microsoft Outlook.
Hornetsecurity - Hornetsecurity offers you comprehensive, integrated managed e-mail security services.