I need a microsoft outlook specialist

Half a year ago register.com mistakenly ended my domain hosting and promptly reinstated it. However, since then my Outlook will randomly prompt for username and password and send/receive fails often. I have had multiple microsoft technicians remote in to my computer but no one can figure out what the problem is. Register.com says it’s an outlook problem, microsoft sometimes alludes to it being an issue with register.com. This problem affects all accounts on the domain using outlook.