Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
imapsync - Console-based utility for migrating IMAP mailboxes.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Gmvault - Backup the emails in your Gmail account.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
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.