Microsoft Exchange Server is recommended for medium to large organizations that require a secure, customizable, and robust email solution, especially those already using other Microsoft products. It is ideal for businesses that prefer on-premises solutions with full control over their data and infrastructure, or for those in regulated industries with specific compliance requirements.
Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
imapsync - Console-based utility for migrating IMAP mailboxes.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
VaultMe - Automagically copy email, contacts, calendars, and files from school, personal, or business accounts. Quick, precise, and ultra-secure. Try it now!
DistroWatch - 9 hours ago - It does include a proprietary software set optimized for business users, students, researchers and developers." A list of features and further details can be found in the company's release announcement.
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.