Microsoft Outlook or Outlook (full name Microsoft Office Outlook since Outlook 2003) is a personal information manager from Microsoft, and is part of the Microsoft Office suite.
Although often used mainly as an e-mail application, it also provides a calendar, task and contact management, note taking, a journal and web browsing.
It can be used as a stand-alone application, but can also operate in conjunction with Microsoft Exchange Server to provide enhanced functions for multiple users in an organization, such as shared mailboxes and calendars, public folders and meeting time allocation.

Versions
Outlook is an integrated application for email, calendaring, tasks, contacts and much more. However, it does not fully support other calendar programs for calendaring or contacts such as iCalendar, CalDAV, SyncML and vCard 3.0. Outlook 2007 claims to be fully iCalendar compliant, however it does not support all core objects, such as VTODO, VJOURNAL. Also, Outlook supports vCard 2.1 and does not support multiple contacts in the vCard format as a single file. Outlook has also been criticized for having proprietary "Outlook extensions" to these Internet standards. Also, for Outlook 2007, Microsoft has replaced the more standards-compliant Internet Explorer-based HTML editing/viewing engine with the one from Microsoft Office Word 2007. Outlook Express is being replaced by Windows Live Mail.
Outlook encourages top-posting by placing the cursor above the quoted text. A proponent of bottom-posting has created an application known as Outlook-QuoteFix to change this default to bottom-posting.

Internet standards
One of Microsoft's goals is for the e-mail client to be easy to use. However, the embedded automation and lack of security features compared to competitors have been repeatedly exploited by malicious hackers using e-mail viruses. These typically take the form of an e-mail attachment which executes on the user's machine and replicates itself by mass-mailing the user's or Exchange server's address list. Examples of such viruses are the Melissa and Sobig worms. Other programs have exploited Outlook's HTML e-mail capabilities to execute malicious code or confirm that e-mail addresses are valid targets for spam. The notoriety of the worms and other viruses has gained Outlook a reputation as a highly insecure e-mail platform.
Unix programmer Bill Joy has suggested that Outlook is insecure largely because it was written in C, making it easy to write programs to exploit it. He also believes the widespread use of Outlook is a major contributing factor in the proliferation of spam

Security concerns
Outlook 2007 was available in retail stores at the end of Jan 2007. A trial is available for download on Office Online:

A to-do bar added to the shell UI that shows a snapshot of the user's upcoming appointments and active tasks for better time and project management.
Improved calendar views that display the tasks due below each day on the week view and supports overlaying multiple calendars.
Integrated RSS aggregator
'Instant Search' through a context indexer based search engine with Windows Desktop Search
Enhanced integration with Microsoft Office SharePoint Portal Server
New programmability features [1]
Ability to publish calendars in Internet Calendar format to Microsoft Office Online or to a WebDAV server