Polymail - Native email app for email productivity.
Roundcube - Web-based IMAP email client
Morning Mail - A new way to manage your mail. Tinder for email.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
HEY - Email at its best, new from Basecamp.