This problem occurs on larger mailboxes. The issue means you can’t really send stuff anymore. Sometimes restarting the PC works.
Googling this is really hard. There is a ton of information on the internet such as checking IMAP folders subscription and subscribing/ unsubscribing.
Eventually we found a Microsoft ‘Answers” article dated 26 June 2023 which worked in our environment.
1. Open Outlook and go to File > Options > Advanced > Send/receive > click on the send/receive button, then click Edit.
2. Find your account and un-check the option “Get folder unread count for subscribed folders“
3. Under Received mail items, check the option “Use the custom behavior defined below“
4. In the window below that displays your IMAP folders just select Inbox and leave the others unchecked
5. Click OK and then exit out of options and perform a send/receive to test if there is any improvement.
Thanks @Katherine Yu MSFT!