Office 365 - Outlook 2007 Support
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This document provides the reasoning behind why Outlook 2007 is not 'officially' supported/recommended in use with Office 365.
The overall result of our testing was that Outlook 2007 was able to perform a vast majority of features and functions, and it kept relative pace with Outlook 2010 and 2013 for overall functionality.
Outlook 2007 works fairly well for the time being; however, Microsoft has a "2 version" support policy. What this means is that they only support a program's current version and one prior version (2016/2013, and best effort support for older versions). Recently Office 365 was upgraded from 'Wave 14' to 'Wave 15' (or in other words, version 14 to version 15), and Microsoft had no intentions of offering support for Outlook 2007 in Wave 15. Due to public outcry and demand, Outlook 2007 was added to the list of supported client versions for Wave 15. Future upgrades to Office 365 may not include support for Office 2007.
Microsoft's current support for 2007
As of mid-May, 2013, Outlook 2007 is past its expected end of general support. Going forward, we do not expect Outlook 2007 to be a supported client by Microsoft in future versions of Office 365. Already, Microsoft's product life-cycle policies indicate that support for Outlook 2007 is now in its 'extended' phase, meaning it will include security patches but won't include any "bug" fixes or address any feature requests.
Outlook 2007 will be covered under UITS's 'best effort' support, but because of this looming end to official product support, we do not list it as officially supported, or even recommended for use with Office 365. This isn't to say the program will not work, because as of right now it seems to work fairly well; we just can't make guarantees it will continue to work as well as it does now further down the road.