There are quite a few see below:
Opera
Free by Opera Software - Mac Windows Linux
Opera is an innovative browser with the features, speed and security that will give you the best browsing experience available. Opera has a built-in e-mail and bittorrent client. It also has unique features such as
Thunderbird
Open Source by Mozilla - Mac Windows Linux
Thunderbird is a free, open source, cross-platform e-mail and news client. The project strategy is modeled after Mozilla Firefox. Advantages compared with the most popular web e-mail application:(a) Excellent Mail-
Google Calendar
Free by Google - Online
With Google Calendar, its easy to keep track of all your lifes important events in one place. You can add events and send invitations effortlessly, share your schedule with friends and family, and search for events you m ...
Evolution
Open Source by The GNOME Project - Mac Windows Linux
Evolution is the official personal information manager and workgroup information management tool for GNOME. It combines e-mail, calendar, address book, and task list management functions.
Rainlendar
Free with limited functionality by Rainlendar - Mac Windows Linux
Rainlendar is a feature rich calendar application that is easy to use and doesnt take much space on your desktop. The application is platform independent so you can run it on all major operating systems.
Lightning
Open Source by Mozilla Foundation - Mac Windows Linux
Lightning brings the Sunbird calendar to the popular email client, Mozilla Thunderbird and the SeaMonkey internet application suite. Since its an extension, Lightning is tightly integrated with Thunderbird or SeaMonkey.
eM client
Free with limited functionality by eM Client Inc. - Windows
eM Client is a Windows based FREE replacement for MS Outlook with e-mails, multiple calendars, contacts and tasks. It allows full synchronization with Gmail or any other server, including IM chat and
Postbox
Commercial by postbox-inc.com - Mac Windows
Based on Thunderbird."A new way to work with emailPostbox offers powerful new ways to find, use, and view email messages and content, organize your work life, and get stuff done.For Windows and Macintosh"" ...
Sunbird
Open Source by Mozilla Foundation - Mac Windows Linux
Mozilla Sunbird is a cross-platform calendar application developed by the Mozilla Foundation, Sun Microsystems and many volunteers.
Windows Live Mail
Free by Microsoft - Windows
Get multiple e-mail accounts in one program – Hotmail, Gmail, Yahoo!* and more. And now Windows Live Mail has a calendar, too. Mail combines the ease of use of Outlook Express, with the speed of Windows Live.
Opera Mail
Free by Opera Software - Mac Windows Linux
Opera offers an email client built right into your favorite browser. Opera Mail powerfully organizes, indexes, and sorts your messages while transforming your Opera web browser into a complete communication tool.
Zimbra Desktop
Free by Zimbra - Mac Windows Linux
The free Zimbra Desktop client allows you to meld the online and offline worlds – storing and synching your email, calendar, contacts, files and documents in the cloud, yet having them locally accessible when on the road.
Claws Mail
Open Source - Mac Windows Linux
Claws Mail is an email client (and news reader), based on GTK+, featuring Quick response Graceful, and sophisticated interface Easy configuration, intuitive operation Abundant features.
The Bat
Commercial by RITLabs - Windows
High-level system of e-communication messages processing with intuitive interface and strong privacy protection.
Banckle Email Server
Free by Usman Sarfraz - Mac, iPad Windows Online
Banckle Email Server is a FREE and reliable business email solution supporting multiple protocols, video email, text and video chat. Considering your business needs, the email communications are SSL secured
Sylpheed
Open Source by Hiroyuki Yamamoto - Mac Windows Linux
Sylpheed is a simple, lightweight but featureful, and easy-to-use e-mail client (mailer, MUA).Sylpheed runs on Unix-like systems such as Linux, BSD, and Mac OS X. It also supports Windows.Sylpheed uses GTK+ GUI toolkit.
EssentialPIM
Free by EssentialPIM - Windows
PIM stands for Personal Information Manager, and it allows you to keep all you information in electronic form. All your appointments, tasks, to do lists, notes and contacts are stored in a graphical and easily accessible..
Try the link below which will give you for llinks to the websites / apps.
http://alternativeto.net/software/microsoft-office-outlook/
I hope this helps.
Rogue ;o)