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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.
Sfletter - SFLetter.com protects your message and attachments by one click. Email protection from interception and unauthorized actions of the recipient.
Gmvault - Backup the emails in your Gmail account.
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imapsync - Console-based utility for migrating IMAP mailboxes.