32-bit Outlook will soon be Large Address Aware

Edit: we've published an additional help article about this update: https://support.office.com/en-us/article/Large-Address-Aware-in-Outlook-2016-ea0913e4-2917-4456-b0ea-935dbd57ef02?ui=en-US&rs=en-US&ad=US 

We'd like to tell you about an an upcoming change in the Office 365 subscription version of Outlook 2016 for Windows that may affect you. Starting in version 1709, the 32-bit version of Outlook will be Large Address Aware (LAA).  What does this mean and how will it impact you?  Read on for more details.

In Windows, normal 32-bit executables have a maximum address space of 2GB. This has been the case for many years.  In those years, however, hardware has moved on and has massively increased its capability.  In particular, machines today have much more memory as well as increasingly higher resolution displays.  In particular, the resolution of displays is the motivating factor for enabling LAA for Outlook.  As displays get larger, the amount of graphics memory required to compose, render, and display applications increases dramatically.  The 2GB limit described above puts pressure on Outlook’s ability to draw all the pixels to the screen—enough pressure that some re-draw glitches might result in some situations.  LAA enabling allows Outlook to have a full 4GB of address space and reduce the chances of those re-draw glitches happening.  Keep in mind that the application won’t necessarily use all of that memory at once, the gains are in having larger contiguous blocks of address space to use in our graphics subsystem.

The casual reader may ask “why should this matter to me?” or “sounds like a good idea, why didn’t you always do this?”  The answer has to do with the history of the 2GB limit for 32-bit processes.  Because 2GB was the highest address that an application would have to utilize, software developers could know that the addresses would only use 31 bits (2GB=2^31).  They could then use the extra “high bit” for any interesting purpose (bookkeeping, for example).  As a result, poorly (or “creatively”) written code may now malfunction in a LAA enabled process.  While LAA Outlook has been extensively tested, there is the possibility that some COM add-ins in Outlook may not be compatible.  Should you experience crashes in Outlook caused by an add-in, the add-in should be disabled, and a warning given when you restart the app.  Please contact the add-in author and request an updated build that is LAA compatible.

All of the above only applies to 32-bit Outlook.  Any customers running 64-bit Outlook are already reaping the benefits of a much MUCH larger address space.

Thanks, and let us know if you have any questions. 

-The Outlook team