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How to deal with outlook performance & inconsistency issues

There is often a small group of people not happy once their mailbox is moved to the cloud or is in the cloud already. This is because they are suddenly dealing with performance and/or inconsistency issues. I have had several calls with our customers about this subject so I finally took the time to write some stuff down. Why do they have these issues? and what can you do about it? I hope this blog can help you bring some improvements into your organization.

What is the problem?

Pre-migration to the cloud you are in a situation where your users often connect via a local LAN connection to the Exchange on-premises servers. If your mailbox is in Exchange Online, things change. Suddenly, Outlook needs to makes connections via the internet and it will take longer to get tasks done (open an email, create an appointment etc.). To improve performance, admins come to the conclusion that caching is the solution for these users, and they are absolutely right. However, after caching is turned on, users see inconsistency problems. For example, meeting updates don’t reflect to all others.


What can you do about it?

There are things that can be done by an admin however, a user can take into account some tips and best practices as well. This is often more difficult as it might change how they did things before. The most important tip I have is that you have to see this as a golden combination of rules in order to improve the total experience. Customers often implement one or a few of these which in the end doesn’t really help.

Do you need full access in the first place?

You might try to solve problems in all kind of ways while basically the user don’t need full access. In many scenario’s what happens is, userA wants to manage the calendar of userB. UserA gets added for full access delegation of the mailbox of userB. If you can limit the amount of accounts, and therefor what you will eventually sync to the .OST file this should be your first step to investigate.

Caching of the shared folders

Management assistants or other “heavy” Outlook users simply don’t work in their own mailbox, so caching their own mailbox will not improve much for them. That is why you need to make sure you are also caching shared folders to the local OST file.






Keep the local .OST file under 10GB

When you cache not only the mailbox, but also the shared folders, it will usually greatly increase the size of the local .OST file. Ideally, you want to keep it under 10GB for the best performance. However, in practice, I see that under 20GB will still be OK. This is when using modern SSD drives and not those conventional hard disk drives.

Enable Online Archives and Retention Tags & Policies

To help make the .OST file smaller, it is highly recommended to enable Online Archives for the mailboxes your management assistants manage. In Exchange Online Plan 2 (E3, E5 etc.) you have a default archive size of 100GB but with a simple Powershell cmdlet you can set the AutoExpand parameter on $true which results in an unlimited storage for this archive.

Combine Online Archives with Retention Policies which moves email from the mailbox after 3-6 months to this archive. These numbers are no hard limits, it all depends on how much email you can keep offline and also keep the .OST file small enough for the best performance. Do note to ask your users how often per day/week they search for emails older than x months as this might impact the amount of email you can move to the archive. Searching for emails in an Online Archive is slower because it all happens online instead of the local SSD drive. The same goes if users need to access their Online Archive from an airplane. No internet = no access to the Online Archive.

Don’t scan the .OST file

I have seen scenario’s with massive .OST files being scanned by the local antivirus client every time Outlook opens or when Outlook runs. This will both result in slow Outlook loading times but also impact the performance within Outlook. It might even break the Outlook profile.

Outlook has limits

100.000 items per folder
5.000 items per calendar folder
500 folders total (not applicable anymore when using a recent version of Office)

If you have Apps for Enterprise (Office) version 1904/1905 (depending on your release), Microsoft redesigned the sync engine being able now to support up to 5000 cached folders. This is all folders and subfolders combined cached to the OST file. Despite of these “new” limits, I have seen that the best practice is to keep the folders to a minimum and use the Outlook search engine to search for emails. Check out this URL for more information about these limits.


What can you do as a user?

It’s not all the admin’s job. Here are some things users can do themselves.

Keep the folders to a minimum

Promoting the “Deleted Items” folder as complete archive solution, making a folder structure based on supplier or even creating folders per year/month and week. I have seen it all and let me tell you it doesn’t help in keeping Outlook fast or trustworthy when synchronizing changes. The search functionality in Outlook is pretty good! Personally, I make folders per year and within it I create 2 subfolders with “sent items” and “inbox”. This might be a potential good strategy going forward. If you really insist in keeping a massive folder structure, then please do so in your Online Archive as these folders are not cached to the OST file and will not have a negative impact in the performance or consistency.

Use the scheduling assistant

When creating many meetings with different people throughout the day, use the scheduling assistant. I often see that the overlay functionality gets used because it is handier or “prettier”.

You know… like this.

However, this results in Outlook building up connections which you can check by opening the Outlook connection status (ctrl+right click Outlook icon).

The thing is, what users don’t know is that these connections are kept active until you close Outlook. This is done on purpose, because if you re-click one of these calendars again, you will have them instant. Now imagine you create meetings all day and it builds up all these connections and keeping them active. Yep, I’ve seen multiple scenario’s with 100-200 active connections to Exchange Online. This doesn’t help in keeping Outlook fast. The scheduling assistant works in a different way and will not do this.



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