Microsoft Exchange Online:

Microsoft Exchange Online is an enterprise messaging solution based on Microsoft Exchange Server 2010 and hosted from Microsoft Data Centres. Exchange Online provides the business with the email security they demand, employees and end users with the level of access the want, and IT staff with the operational efficiency they need packed in a resilient, highly available, enterprise solution.

Exchange Online is one of the components found within the Office 365 suite of products and provides access to email, calendar, contact and task functionality from virtually anywhere at any time. Exchange Online can be delivered through many devices and across many platforms to ensure easy adoption and usability to the end user.
Features of Exchange Online:
 

• Work from virtually anywhere: You can now work from anywhere, with automatic updates to emails, calendars and contacts on the devices you use the most, including, PC's, Macintosh computers, iPhones, Androids, Blackberry's, Windows Mobiles and Windows Phones. 
• Easily managed emails: Office 365 provides users with a 25gb mailbox, letting them send email messages up to 25mb. Whether you are connected to the Internet at home of in the office or you are working offline, you can connect to Microsoft Outlook 2010 or Outlook 2007 and use all of the rich, familiar Outlook functionality you already know and use. 
• Simplify Scheduling: Sharing calendars and viewing them side by side allows users to see colleagues availability and suggested meeting times from their calendar, making meeting scheduling easier to do. You can now access your email, calendar and contacts from any web browser while you keep the rich, familiar Outlook experience with Microsoft Outlook Web App. 
• Business-class security: Microsoft Forefront Online Protection for Exchange helps protect your organisation from spam and viruses and includes multiple filters and virus-scanning engines.

The Key benefits of Exchange Online are:

• Industry-leading, always up-to-date antivirus and anti-spam solutions
• Disaster recovery though continuously-replicated, geo-redundant data centres that are third-party certified to international standards
• Robust management tools that keep IT administrators in control of user permissions, distribution lists and security policies similar to a sophisticated on-premises deployment
• Familiar end-user experience with the same Microsoft Outlook and Office programs you know, ensuring a minimal learning curve
• Interoperable with most devices, including PCs, Macs, BlackBerry, Android, Windows
• Phone and iPhones as well as all major web browsers
• Large 25 GB mailboxes and the ability to send attachments up to 25 MB
• 24/7, IT-level support over the phone, the web or email
• Financially-backed, guaranteed 99.9% uptime Service Level Agreement

Download the Exchange Online technical overview here.