Followup.cc - FollowUp for email is a lightweight productivity suite that keeps your conversations and tasks from slipping through the cracks. Set perfectly timed reminders that arrive at the top of your inbox, right when you need them.
imapsync - Console-based utility for migrating IMAP mailboxes.
FollowUpThen - The simplest way to schedule an email reminder
MailStore - MailStore Home - A 100% free single-private-user desktop solution
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
audriga Email and Groupware migration - Easily migrate your email, contacts, calendar items and tasks to your new provider or server. No software installation is required.