Kontact - Kontact is the integrated Personal Information Manager of KDE.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Google Contacts - Google Contacts is Google's contact management tool that is available in its free email service Gmail, as a standalone service, and as a part of Google's business-oriented suite of web apps Google Apps.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Kontact - KAddressBook - KAddressBook stores all the personal details of your family, friends and other contacts.
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.