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.
Backblaze - Backblaze's remote backup automatically backs up your data to our secure datacenter.
Gmvault - Backup the emails in your Gmail account.
SpiderOak - SpiderOak makes it possible for you to privately store, sync, share & access your data from everywhere.
imapsync - Console-based utility for migrating IMAP mailboxes.
CrashPlan - CrashPlan for Small Business backup software offers the best way to back up and store business & enterprise data securely - offsite, onsite & online in the Cloud.