Outlook 2007 was the first Outlook to switch from Internet Explorer HTML rendering to Microsoft Word 2007 HTML rendering. This means a number of HTML and CSS items are no longer supported due to security reasons.

The consequence of this mostly affects those publishing newsletters, which frequently use intricate HTML and/or CSS to form their layout, for which existing code will not render correctly. For example, the removal of a ("form") tag means that forms can no longer be embedded in e-mail.

This switch can be seen to mark a change in philosophy on the Outlook product, making it intentionally incompliant so that only the most basic of presentational elements remain.

Calendar compatibility

Outlook 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.