Outlook (classic) mailbox error

Question

I'm using Outlook (classic), version 2601 (build 19628.20214 click-ro-run), and until Thursday evening it has been running very well. I have access to 14 shared mailboxes within Outlook.

Since Friday morning (20-02-2026) it has been extremely slow and now I get this error whenever I want to create a meeting or delete an email etc:

I am no where close to reaching the maximum size of my mailbox:

I have a colleague who has reported the same error. He has a used appr. 55 GB of 100 GB available but access to only a few shared mailboxes.

I test the new Outlook on a regular basis so I tried again Friday and it runs smooth without the errors mentioned. So it seems like it's an Outlook (classic) issue.

Any ideas what has happened?

Flawed update of some kind?

Or the Microsoft way to tell us that we need to stop using the Classic edition and instead only use the New edition ...?

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Mod moved from : Microsoft 365 and Office | Other

Answer 1

Hello Morten Fly
Thank you for posting question to Microsoft Q&A Forum.
Update as for February 24, 2026:

• By adding this value within Registry Editor: HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST MaxLargeFileSize REG_DWORD (102400) - this would temporarily increase the size limit of *.ost file - This solution is reliable until the growth of all mailboxes reaches to this new limit.

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Based on my understanding, what’s happening here is mostly down to how Outlook (classic) handles shared mailboxes when Cached Exchange Mode is enabled. Even though the shared mailboxes were added automatically through Auto‑Mapping (not manually in Account Settings), Outlook still treats them the same way internally. By default, Outlook downloads and caches all shared mailboxes into a single local .ost file together with the primary mailbox. When you have access to many shared mailboxes, Outlook doesn’t just cache message headers — it caches entire folder structures and mailbox data. Over time, this causes the OST file to grow very large and increases the number of objects Outlook has to keep track of. Once certain internal limits are reached, Outlook can become slow or unstable and may start showing misleading errors such as “mailbox size” issues, even though the actual Exchange mailbox quota has not been exceeded. This should be expected behavior with shared mailboxes in Cached Exchange Mode, especially when many folders or mailboxes are involved.
Could you try to turn off "Download shared folders" to see if this still helps to load the shared mailboxes' emails when you need to access it but not save a local copy of this into your local computer's Outlook profile.
Within Outlook, click on File > Account Settings > Account Settings again > double-click on your email > More settings > Advanced > Turn off "Download shared folders".

Please try this and let me know if this works for you.

I am looking forward to your response.

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