thunderbird

SOGo is groupware server with a focus on scalability and open standards.

MozBackup is a simple utility for creating backups of Mozilla Firefox, Mozilla Thunderbird, Mozilla Sunbird, Flock, SeaMonkey, Mozilla Suite, Spicebird, Songbird and Netscape profiles.

MailStore Home - A 100% free single-private-user desktop solution

Betterbird, a soft fork of Mozilla Thunderbird, improves user experience with advanced features, bug fixes, security patches, and multi-row descriptions similar to Outlook. Compatible with multiple platforms and can be used alongside Thunderbird, sharing the same profile.