Today my friend had a problem with BT Mail taking a long time syncing with Office Outlook.
The BT Mail account is set up with IMAP.
Office Outlook seemed to freeze when syncing with BT Mail, is this an Outlook issue or is the issue with BT Mail?
I had trouble of this kind due to a corrupt .pst file - there is a piece of MS software that checks it. Worth a try.
It used to happen a lot when I was on POP email, which made me move to 365 and IMAP.