Gmvault - Backup the emails in your Gmail account.
Barracuda Email Protection - Barracuda Essentials offers advanced Office 365 security features including email scanning, archiving, backup & recovery, and real-time phishing defense.
MailStore - MailStore Home - A 100% free single-private-user desktop solution
Athena Archiver - Athena Archiver Email Archiving Solutions provide enhanced disaster recovery options by enabling automatic off-site email storage and data replication to a mirror site.
SysTools Gmail Backup - Backup Gmail Emails, Contacts, Calendar, and Google Drive Storage File by SysTools Gmail Backup Solution
Thexyz - Secure premium email service with mobile sync apps for calendar, contacts, tasks, notes and FISMA compliant email archiving.