Unable to re-add MS365 Shared Mailboxes after removing them (Delegation Search fails)

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Hi everyone,

I am using eM Client (Pro Version) on Windows 11 and I am encountering a specific issue with Microsoft 365 Shared Mailboxes.

My Setup:

  • Microsoft 365 (2 licensed primary mailboxes + several Shared Mailboxes).
  • Google Account (via standard Google login).
  • Several additional IMAP accounts.
  • The Shared Mailboxes are accessible via my MS365 credentials.

The Issue: I am unable to re-add a Shared Mailbox via the “Delegation” tab if that mailbox was previously added and then removed.

Steps to Reproduce:

  1. I had a Shared Mailbox (let’s call it [email protected]) setup in eM Client via Accounts > Exchange Web Services > Delegation. It was working perfectly.
  2. I had to remove this shared mailbox from the list in the Delegation tab.
  3. I restarted eM Client.
  4. I attempted to add the mailbox back ([email protected]) via the same path: Accounts > Exchange Web Services > Delegation > Add.
  5. Result: The search function yields no results.
  • I tried searching by the exact email address.
  • I tried searching by partial strings (e.g., “num”, “number”).
  • The search returns other users/mailboxes, but explicitly not the one I just deleted.

Troubleshooting / Observations:

  • Reproducibility: I tested this with a second working Shared Mailbox. I removed it, restarted, and tried to add it again. The behavior was identical: eM Client can no longer “find” it in the directory search.
  • Cross-Check: I verified this in Microsoft Outlook. Both shared mailboxes are visible and fully functional there. The accounts definitely exist on the server side.
  • Multiple Accounts: I tried searching for these shared mailboxes using my second licensed MS365 account within eM Client (under Delegation). The result is the same: the specific mailboxes that were removed are invisible to the search in eM Client.

It seems like eM Client is locally blocking or failing to index these specific addresses after they have been removed once.

Has anyone experienced this or is there a way to clear a specific cache for the Global Address List/Delegation search?

Thanks for your help!