Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
ReferralMD - ReferralMD is a HIPAA-compliant, referral management CRM software; workflow, e-consult, fax management, patient access, provider directory, and analytics
Geary - Geary is an email application built for GNOME 3.
Cerner Direct Referrals - Referral Management
Evolution - Apps/Evolution - GNOME Wiki!
Hippocrate - Patient Engagement