Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
DuoCircle - DuoCircle provides enterprise spam filtering and email management solutions.
MailClark - The Slack bot for external communications
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
Maid - Be lazy. Let Maid clean up after you, based on rules you define.
Aid4Mail - Aid4Mail is a software specialized in migrating a massive range of email programs, webmail services and mailbox formats etc.