imapsync - Console-based utility for migrating IMAP mailboxes.
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.
MailStore - MailStore Home - A 100% free single-private-user desktop solution
FollowUpThen - The simplest way to schedule an email reminder
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.
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.