Large mailboxes & managing multiple mailboxes…. Ohhhh My!

Recently I got involved with a client who had made the jump from their on premise Exchange deployment to Office 365 Exchange Online. Awesome! Only this client started experiencing some serious performance issues for their executive assistants who managed multiple mailboxes. Think to the tune of being able to get a coffee in the Starbucks down the road when changing folders…

Turns out they used to run outlook in ‘Online’ mode and now switched to ‘Cached’ mode, which happens to be the preferred usage scenario for outlook running against a Microsoft Exchange environment…

So what happened?

That’s the 1-million-dollar question, isn’t it? In this case the customer ran in to some serious client IOPS issues as the combined size of all the mailboxes bloated the OST file (the local outlook cache!) somewhere between 30GB and 50GB. Considering the limit for OST files in outlook 2010 and outlook 2013 is actually 50GB, as well as that outlook OST access is random, that causes some heavy disk usage! Something the client computers with their 7200rpm spindles just couldn’t handle…

So out comes the magic stick and we solve the problem. But along the way, we realized there just isn’t any good material out there to assess when to run in to ‘Online’ mode vs when to run in ‘Cached’ mode.

So let’s start at the beginning shall we?

Outlook Cached mode is the preferred connection method. It provides us with a local copy of the user’s mailbox (the OST file), against which all actions will be performed. Any changes to this OST file will periodically be uploaded to the Exchange server’s master copy (yes, the data in the mailbox on the Exchange server is referred to as the ‘Master Copy’)! That means anything you do will be delayed, but you’re not dependent on a network connection to be present.

So cached mode typically has been recommended for the following reasons:

· You’re a mobile worked that needs access to your emails, even if you are not connected to a network.

· You have a high latency connection (greater than 500ms).

· You want to reduce load on the Exchange servers.

Now that last one is an interesting one. My arguments and battles have been fought over it, and it was most definitely the case with Exchange 2003, but in the more recent versions, the statement is that there is no significant server impact between ‘Cached’ mode and ‘Online’ mode…

Outlook Online mode, however, does have its uses!

Kiosk users: You really don’t want to have an offline copy of a user’s mailboxes on kiosk computers. That’s just a nightmare waiting to happen…

Large mailboxes: Large mailboxes (Think 10GB+) or multiple mailboxes in a profile that would result in an OST file (that local copy of your mailbox!) larger than 10GB have the recommendation of running in ‘Online’ mode.

VDI or RDS environments where your file servers cannot keep up with the strain from having multiple users caching their mailboxes in outlook 2013 carry the recommendations of running in ‘Online’ mode.

Lastly, heavily regulated environments where data can never be stored locally also carry the recommendation of running in ‘Online’ mode… Although you can think of using ‘Cached’ mode if you are running drive level encryption, which will result in a more robust solution. Obviously check if your regulatory agency allows this configuration!

OST File size recommendations

OST file sizes cause lockups! Microsoft even has a KB support article about it: Which basically comes down to this:

· Up to 5 GB: This file size should provide a good user experience on most hardware.

· Between 5 and 10 GB: This file size is typically hardware dependent. Therefore, if you have a fast hard disk and lots of RAM, your experience will be better. However, slower hard disk drives, such as drives that are typically found on portable computers or early-generation solid-state drives (SSDs), experience some application pauses when the drives respond.

· More than 10 GB: When the .ost file reaches this size, short pauses begin to occur on most hardware.

· Very large (25 GB or larger): An .ost file of this size increases the frequency of short pauses, especially while you are downloading new email messages. However, you can use Send/Receive groups to manually sync your mail. For more information about Send/Receive groups, see the "Are you synchronizing many RSS feeds?" section.

But what about that slider?

You mean that awesome new slider introduced in Outlook 2013, limiting the amount of data you download to a certain time frame? Yup, that is an awesome thing to think of! Unfortunately, that slider only applies to you Primary Mailbox. Meaning that you will only download that timeframe period for your actual mailbox, and for all the shared mailboxes or public folders you have configured you will still download all the data if you choose to run them in ‘Cached’ mode…

So what are my options?

Well you could simply choose to run in ‘Online’ mode only. That will cut out any disk performance issues but will put a heavy emphasis on your network connection. You’re likely to still see delays as every action you take (and every move you make) is going to be performed against the Exchange server and needs to be confirmed by the Exchange server before you can move on.

You could run everything in ‘Cached’ mode, resulting in all actions being performed against the local copy of your mailbox. This generally results in a smoother experience for the end users, with obvious exceptions if you have a large mailbox or are managing multiple larger mailboxes culminating in an OST file size of 10GB+.

If you are in that last situation you have a number of options which you could think of:

Run your primary profile in ‘Cached’ mode but not your shared mailboxes. This might actually work for specific scenarios but for most customers, this will not be the route to take.

Run both your primary profile and your shared folders in ‘Cached’ mode. If you will experience disk issues due to a large OST file you can decide to implement retention policies limiting the amount of data stored in the mailboxes, moving older mail items to an archive mailbox.

That obviously requires you to have a connection if you need to access to older items as they are never cached, so you can’t access them offline.

Use that slider in conjunction with synchronization filters. These filters are applicable to folders (right click the folder, select properties and go to the ‘Synchronization’ tab) and allow you to set the same limit to the shared mailboxes & folders that you set up against the primary mailbox.

Do note, however, that the slider not only applies to your primary mailbox, but also excludes the following folders in your primary mailbox:

· Calendar

· Contacts

· Tasks

· Journal

· Notes

· Outbox

· Shared or delegated

Use faster disks: OST File access can be considered highly random. Because of that regular spindles are going to choke up on the larger variety of OST files. Using SSD drives is going to considerably impact that end user experience in a positive manner!

And there you have it!

Far from being a perfect solution, this should help get you started in thinking and possibly resolving some of those slow client machines that are using outlook in combination with large OST files.

In this case I also want to call out Ryan Phillips ( for working with me on this customers issue and being an all-round bad ass!

If you’re interested in reading more on this topic I can highly recommend the technet article on ‘Choose between Cached and Online mode in outlook 2013’ (


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