Outlook Send/Receive Hangs Frequently

Hi Experts,

My client has 2 accounts in Outlook, IMAP & Exchange, and a lot of times when she does Send/Receive, it hangs but it does not freeze.

First, my suspicion was her IMAP account, which is hosted on a VPS, but then when I Send/Receive Exchange-only it also hangs. I am on the same Exchange server, and it does not hang, so I'm thinking this too is influenced by the IMAP.

For now, the client does not want to migrate IMAP to Exchange , unless I can really prove that this is it.

I was also thinking to create an IMAP account on that same domain for myself, add it to my Outlook, and write a quick script to send that email 200 messages, but I feel it will work.

The second observation is, I closed down her Outlook to check the email accounts under Control Panel > Mail, which both passed the test. However, after that, I could not restart Outlook. She also complained that Outlook is also slow to start, so I waited 10 minutes and nothing. I had a look in Task Manager, and saw an X with a gray circle next to Outlook. I ended the process tree, clicked Outlook and it opened in 5 sec.

She keeps restarting, but it does not help. I noticed in Windows 10 that restart does not close everything. is this true?

Lastly I noticed that sometimes her disk goes up to 100%, and I did everything in this article. Nevertheless,  her C: is 617GB free of 913GB.

Thanks