Spark Mail - Spark helps you take your inbox under control. Instantly see what’s important and quickly clean up the rest. Spark for Teams allows you to create, discuss, and share email with your colleagues
imapsync - Console-based utility for migrating IMAP mailboxes.
Newton Mail - Newton is a fully featured mail client available for Android, iOS, macOS and Windows.
Gmvault - Backup the emails in your Gmail account.
Aqua Mail - Android Mail Client.
Cryoserver - Cryoserver is an all-in-one email archiving solution that empowers you to preserve your email in a tamper-evident archive, making you transform your data into a useful archive for everyday use.