MailStore - MailStore Home - A 100% free single-private-user desktop solution
Gmvault - Backup the emails in your Gmail account.
Mail Backup X - Your one stop mail backup and archiving tool for Mac.
MailSteward - MailSteward : The Ultimate E-mail Manager for Mac OS X
CubeBackup - A Self-hosted backup solution for Google Workspace
Piler - Piler is an open source email archiving solution with all the necessary features for your enterprise