Option 1 (Recommended): Create a New Outlook Profile

  1. Close Outlook completely.
  2. Open Control Panel → Mail (Microsoft Outlook).
  3. Click Show Profiles.
  4. Click Add.
  5. Give the profile a name (e.g., Outlook-New).
  6. Sign in with your Microsoft 365 account.
  7. Select Prompt for a profile to be used (temporarily).
  8. Open Outlook using the new profile.
  9. Allow Outlook to fully synchronize.

If the missing emails appear, the original profile/OST was likely corrupted.

Option 2: Rebuild Only the OST Cache

This is also generally safe because OST files are just cached copies of mailbox data stored on Exchange Online.

Before you start

  • Verify the emails exist in Outlook Web Access.
  • Ensure you're connected to Microsoft 365/Exchange and not using POP.

Steps

  1. Close Outlook.
  2. Press Windows + R.
  3. Enter:
Plain Text
%LOCALAPPDATA%\Microsoft\Outlook
`
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  1. Locate the .ost file for your mailbox.
  2. Rename it:
Plain Text
Mailbox.ost → Mailbox.old
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  1. Start Outlook.
  2. Outlook will create a new OST and download mailbox contents from the server.

For large mailboxes (such as many HUC mailboxes), the initial sync can take quite a while, so don't worry if folders appear incomplete at first.

Before Rebuilding, Check One Setting

If the issue is "There are more items on the server" and the folder contains older mail:

  1. Outlook → File → Account Settings → Account Settings
  2. Double-click your account.
  3. Check Download email for the past.
  4. Set it to All.
  5. Restart Outlook.

I've seen this setting cause exactly the behavior you're describing, especially when looking in old instruction/archive folders.

If This Is a Shared Mailbox

Do not immediately rebuild the profile.

First:

  • Open Outlook on the Web.
  • Open the shared mailbox.
  • Verify the messages exist there.

Many shared mailbox issues are actually synchronization problems with the shared mailbox itself rather than the user's OST.

For your environment, I'd try:

  1. Outlook Web (verify mail exists).
  2. Set Download email = All.
  3. Create a new Outlook profile.
  4. Only then rebuild the OST directly if needed.

That sequence has the lowest risk and preserves a rollback option.