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Comprehensive Guide on Addressing Slow / Sluggish Outlook Performance to Microsoft Exchange

Identifying and Solving Performance Issues with Microsoft Outlook and Exchange

By Rand Morimoto on Fri, 07/16/10 - 6:57pm.

For organizations using Microsoft Outlook and Exchange server with users complaining that Outlook is REALLY slow and sluggish, and there are performance issues with Outlook 2003, Outlook 2007, even Outlook 2010, there are known problems and known fixes to the performance problems. I've been meaning to write this blog post for years, never found the time, but after getting FIVE calls in the past 48-hours for companies (large and small) having a LOT of users complaining about Outlook performance issues, I finally broke down and wrote this guide.


This is not a guide based on opinions, theories, or mere observations, but I took the time to actually grab Microsoft TechNet articles and other authoritative documents to back the information I'm providing as I find there's a lot of "opinions" out on the Web, lots of incorrect information, or stuff that you just can't take to management and say that some unknown guy said the problem is X, and the solution is Y.  In addition to authoritative reference materials, I'm also going to explain (hopefully in clear English) why the problem exists as well as common steps to FIX the performance issues.  And if that's still not good enough, I have an FAQ section at the bottom to address anything I didn't address in the main body of this post.


Here you go…


Exchange 2003

If you're running Exchange 2003 still and your Exchange server is pretty maxed out (old servers, old hard drives, you've added a lot of users to Exchange since the server was originally implemented, etc), then Exchange 2003 is commonly a place to start for consideration to improve user experience and performance.  Exchange 2003 was a 32-bit environment that only supported a maximum of 4gb of RAM in the server, was not extremely efficient in how it read/wrote messages to disk, and as email demands have grown (users sending large attachments, mobile phones slamming the server on top of desktop users hitting the server, users keeping lots of email, etc), Exchange 2003 was just not built to support the current world of email.  Exchange 2007 and Exchange 2010 are 64-bit environments, adding more memory provides a LOT more buffer space for the management of messages, and with Exchange 2010 a shift from a random read/write Exchange database to one that defrags and writes messages sequentially makes the latest Exchange servers significantly more efficient (5-10 times or more efficient) than Exchange 2003.  More benefits of Exchange 2007 up on:, and workshops I've done on improvements in Exchange 2010 up on   (sorry, I said I'd focus on fact, not opinions, the rest of the bullets in this post gets more factual, but I had to start with recommending orgs to get off Exchange 2003 because the rest of what I note will have more impact if the backend is on Exchange 2007 or preferably Exchange 2010)


Outlook 2003 / 2007 Limitations of Mailbox Size

Original releases of Outlook 2003 and Outlook 2007 have limitations on mailbox size (which is 2gb), although Outlook users can continue to grow their mailbox as large as they want, Outlook starts to slow down when the user's mailbox exceeds 2gb.  See Microsoft KB Article    As this article notes, "Microsoft Office Outlook 2007 momentarily stops responding during typical operations. This includes when you read e-mail messages, move e-mail messages, or delete e-mail messages."  Microsoft did release an update to Outlook 2007 ( so you can theoretically extend your mailbox up to 75gb (and Outlook 2010 also supports this out of the box 75gb capability), however the larger the mailbox (beyong 2gb, even with an update Outlook 2007 or Outlook 2010), the slower the performance you will experience.  See the next note on "Too Many Messages in a Folder" for more information.


Technical Background:  The 2gb limit is a 32-bit filesystem limitation, the operating system cannot handle contiguous files greater than 2gb as a single file, it has to break the file up into smaller blocks to manage the file.  So despite the Outlook 2007 or 2010 support for greater than 2gb OST file (which is where Cachemode Outlook stores the offline data), maintaining a mailbox with less than 2gb of storage will optimize the filesystem access to Outlook (ie: will allow Outlook to run more efficiently, and thus not as "sluggish" or "slow")


Solution:  For users that need more than 2gb of storage, implement an archiving solution (like Symantec Enterprise Vault, Iron Mountain NearPoint for Exchange, etc) or even Exchange 2010 Archiving that is built into Exchange 2010 to keep the user's primary mailbox under 2gb, and allow the archive to hold the user's excess mail.


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