Outlook Classic Slow Accessing Uncached Shared Mailboxes

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Paul1977

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#324862 4-Jun-2026 09:25

Over the past few days Outlook Classic has become orders of magnitude slower accessing uncached shared mailboxes. Some users access many mailboxes, and some of them are very large - hence why we are only caching the users primary mailbox.

Access uncached shared mailboxes has always been a little slow, but at the moment it is horrendous and bordering on unusual in some cases.

When I last looked at the "New" Outlook it was still pretty terrible, so we'd been sticking with "classic".

Microsoft isn't reporting any issues, so was just wondering if anyone else has noticed this?

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freitasm

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#3500140 4-Jun-2026 09:42

I have questions.

What ISP?
Where are the mailboxes? In-house Exchange, Exchange on-line, Gmail, Google Workspace?
If using Exchange on-line, where is the datacentre?
Is this using Wi-Fi, Ethernet or mobile data?
Are there any PST files being used, or just OST files?
Any files on a network share?
What kind of computer? Storage? Memory?
If this is a laptop, is the storage hardware set to Enable write caching and Turn off Windows write-cache buffer flushing?
How is Windows Indexing set up? Is it working on the mailbox?

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nztim

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#3500142 4-Jun-2026 09:44

Simple fix

Remove Read and Manage permission to your shared mailbox though the M365 Admin Center (Leave Send As Alone)
Install the exchange online management PowerShell module, Instructions Here
Connect to Exchange Online (use same link above)
Execute the command Add-MailboxPermission -Identity "shared@mailbox.name" -User "user@mailbox.name" -AccessRights FullAccess -AutoMapping $false
WAIT 45 MINUTES
Add to outlook manually as its own mailbox (not as additional mailbox under advanced)
When prompted for credentials go sign in as another user and sign in as the user who has permission to the mailbox

This creates its OST file for the shared mailbox and completely removes the outlook lag

Any views expressed on these forums are my own and don't necessarily reflect those of my employer.

freitasm

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#3500143 4-Jun-2026 09:55

Ok... But why? Is there something about this recipe, and why the 45-minute wait? To allow some property cache to clear?

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nztim

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#3500152 4-Jun-2026 10:21

freitasm:

Ok... But why? Is there something about this recipe, and why the 45-minute wait? To allow some property cache to clear?

Outlook never caches auto mapped mailboxes properly, and even when it has done the OSTs blow out as they have a limit of 50GB

Creating a separate OST per mailbox connected to outlook resolves that (both space and caching issues)

The 45-minute wait is because Microsoft is so slow and adding/removing permissions on EXO, I allow 20 minutes to remove, 20 minutes to add back and 5 as buffer

in the days of On-Prem it was instant

Any views expressed on these forums are my own and don't necessarily reflect those of my employer.

Paul1977

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#3500157 4-Jun-2026 10:27

freitasm:

I have questions.

What ISP?
Where are the mailboxes? In-house Exchange, Exchange on-line, Gmail, Google Workspace?
If using Exchange on-line, where is the datacentre?
Is this using Wi-Fi, Ethernet or mobile data?
Are there any PST files being used, or just OST files?
Any files on a network share?
What kind of computer? Storage? Memory?
If this is a laptop, is the storage hardware set to Enable write caching and Turn off Windows write-cache buffer flushing?
How is Windows Indexing set up? Is it working on the mailbox?

Sorry, should have been more specific:

Exchange Online Australia East
Most on Ethernet and standard speed tests are good
Just OSTs
OSTs stored on local PC
Various laptops, but the issue is the same across the board for everyone and started at the same time
Windows Indexing will be whatever the default settings are

It's impacting all of our users, and just started in the past few days.

EDIT: ISP is Spark.

Paul1977

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#3500160 4-Jun-2026 10:39

nztim:

Simple fix

Remove Read and Manage permission to your shared mailbox though the M365 Admin Center (Leave Send As Alone)
Install the exchange online management PowerShell module, Instructions Here
Connect to Exchange Online (use same link above)
Execute the command Add-MailboxPermission -Identity "shared@mailbox.name" -User "user@mailbox.name" -AccessRights FullAccess -AutoMapping $false
WAIT 45 MINUTES
Add to outlook manually as its own mailbox (not as additional mailbox under advanced)
When prompted for credentials go sign in as another user and sign in as the user who has permission to the mailbox

This creates its OST file for the shared mailbox and completely removes the outlook lag

Wouldn't just ticking the "Download shared folders" box in the account setting in Outlook Classic start caching the other mailboxes to the existing OST, if caching the shared mailboxes was the goal?

But caching the shared mailboxes isn't the goal. Some users have access to a lot of mailboxes, and when we did cache all of them it caused its own issues as it was constantly updating the OST causing slowdowns and delays. Maybe separate OSTs would help with that, but ideally we don't want to cache the shared mailboxes.

The problem is that it was perfectly useable up until a few days ago, but has suddenly and dramatically gotten worse. I don't think the problem is on our end, so I'm really just trying to determine if it is being experienced by anyone else or whether it might be an issue specific to our Exchange instance before raising it with Microsoft (which from past experience is a long and painful process).

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nztim

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#3500166 4-Jun-2026 10:59

Paul1977:

Wouldn't just ticking the "Download shared folders" box in the account setting in Outlook Classic start caching the other mailboxes to the existing OST, if caching the shared mailboxes was the goal?

Doing that is stupid, your OST will get 50GB and stop

do the users own mailbox work perfectly fine as cached?

If so, the shared mailbox will perform just like the user mailbox when it has its own OST

Any views expressed on these forums are my own and don't necessarily reflect those of my employer.

Paul1977

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#3500172 4-Jun-2026 11:16

nztim:

Paul1977:

Wouldn't just ticking the "Download shared folders" box in the account setting in Outlook Classic start caching the other mailboxes to the existing OST, if caching the shared mailboxes was the goal?

Doing that is stupid, your OST will get 50GB and stop

do the users own mailbox work perfectly fine as cached?

If so, the shared mailbox will perform just like the user mailbox when it has its own OST

"Download shared folders" is the default setting, we disable it via GPO. So stupid or not, we aren't doing it.

nztim

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#3500174 4-Jun-2026 11:19

Paul1977:

"Download shared folders" is the default setting, we disable it via GPO. So stupid or not, we aren't doing it.

What about the answer to my second question? "do the users own mailbox work perfectly fine as cached?"

Any views expressed on these forums are my own and don't necessarily reflect those of my employer.

Paul1977

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#3500194 4-Jun-2026 12:14

nztim:

What about the answer to my second question? "do the users own mailbox work perfectly fine as cached?"

Sorry, yeah they do. But having everyone change their Outlook settings to re-add the shared mailboxes differently wouldn't be my first choice as it's not a small number of users - and the problem only started a couple of days ago after years of working fine.

With the current approach I assign permissions and the mailbox automatically populates in Outlook, so very easy.

I've raised a support ticket with Microsoft, but I'm not overly confident based on past experience with them.

nztim

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#3500201 4-Jun-2026 12:40

Paul1977:

With the current approach I assign permissions and the mailbox automatically populates in Outlook, so very easy.

I've raised a support ticket with Microsoft, but I'm not overly confident based on past experience with them.

Easy for admin and zero touch with the user but a horrible experience for issues like this

Good luck with MS mate.

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#3500274 4-Jun-2026 16:43

For a long time i've heard the advice that shared mailboxes should be used via OWA if at all possible, especially with EXO.

I thus avoid using them in Outlook and consequently don't have too many issues.

I do use them from time to time in Outlook, but not often.

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