Lotame - Make your data actionable, learn about your most valuable customers, improve ROI by targeting the right audience and increase relevance across screens.
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.
V12 Data - V12 Data offers rich data sets with verified addresses and emails for personalized marketing campaigns.
Gmvault - Backup the emails in your Gmail account.
Google Audience Center - 3 votes and 6 comments so far on Reddit
imapsync - Console-based utility for migrating IMAP mailboxes.