Emails downloading issue

One of my user was facing an issue when all Inbox emails are showing/available on Webmail but not downloading after October 2021 till April 2022 in outlook. After applied all possible solution like, create new outlook profile, reinstall MS office etc... finally I was found that on 20th Oct 2021 an email with "No Subject" placed in Inbox and after delete this email from webmail, all remaining emails from Oct'21 onwards start to download in Outlook....and finally all emails have been sync with outlook.