Apple Mail Issue with Optimum on iMAC

Current e-mails from my provider, Optimum, are disappearing from my Apple e-mail on my iMac. They populate in my inbox but just disappear within a few minutes. When I close the mail and reopen it, all mail in the account reloads, but the current e-mails just disappear again. The last two months of e-mails are impacted. Mail from the same service provider are working without problem on my iPhone and iPad.

Have been working with Apple online support. This includes, updating IOS, deleting and re-adding the mailboxes, mailbox repair and removing / adding back iCloud account. It was suggested to contact Optimum directly. I will do that but don't have much faith in their technical support. Has any Optimum user had this issue?