Roundcube - Web-based IMAP email client
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
HEY - Email at its best, new from Basecamp.
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
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
MailClark - The Slack bot for external communications