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 organisation, such as shared mailboxes and calendars and meeting time allocation.
Versions for Microsoft Windows include
One of Microsoft's goals is for the email client to be easy to use. Unfortunately, the embedded automation features and the disregard of security features (which would inconvenience a non-sophisticated user), were exploited by numerous people leading to a phenomenon of email viruses, typically an email attachment executing on the user machine, replicating itself via mass-mailing to the machine address list. Examples of such viruses are the Melissa worm and the Sobig worm. Bill Joy has stated an opinion that the use of Outlook is a contributing factor to network insecurity because—due to its being written in C—viruses propagate so easily with it.
As part of its Trusted Computing initiative, Microsoft has recently took some corrective steps to fix Outlook's reputation as an insecure platform in its latest incarnation, Office Outlook 2003. As of December 2003, it is too soon to tell how successful these changes have been.