ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
imapsync - Console-based utility for migrating IMAP mailboxes.
Tutanota - Get your encrypted mailbox for free.
Gmvault - Backup the emails in your Gmail account.
Mailvelope - Mailvelope open source project. OpenPGP in your browser.
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.