Mailwasher - Over 8 million people use MailWasher as their spam filter
Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates
SPAMfighter - SPAMfighter and Microsoft have built the strongest and most effective anti spam filter for Outlook, Windows Live Mail, Outlook Express & Thunderbird
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Spam Reader - Spam Reader is a free anti-spam add-on for Microsoft Outlook.
Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.