Email inbox in one Mac Mail account shows email from only the last 10 days

We have 5 different Mac Mail accounts in the family, all to which have had thousands of past emails. All of a sudden, one of the accounts only shows emails received in the last 10 days. The missing emails were not deleted and are not in the trash. I looked at Time Machine and when I pick a date in the past it shows only 10-20 mails recently received as of the selected past date. The server is xFinity, which says it is the 3rd party email client that is the problem. This makes sense to me given that Time Machine is also now only showing 10-20 emails in the single affected email account, while the other accounts have thousands of past emails. The "sent" folder is not affected, only the in-box. Thanks for your help.