imapsync - Console-based utility for migrating IMAP mailboxes.
FastMail - Email, calendars, and contacts to highest standards of security, privacy and reliability.
MailStore - MailStore Home - A 100% free single-private-user desktop solution
Loop Email - Emailing for teams: Organize your Inbox into channels 💌
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.
Spike - Spike solves businesses' communication chaos by bringing together email, team chat, collaborative docs, and meetings, into one feed.