Next time you are at the Frontier site either move the problem email out of the inbox and into another folder or delete it so your other messages can get downloaded into Thunderbird.

when I delete they return at intervals ans keep repeating

If you delete it at your providers site and they keep reappearing that is not a Thunderbird issue. Seek help from your provider.

The problem is only with with Frontier email when entered using Thunderbird. When using internet explorer for my Frontier email i get all my messages..

Maybe someone else can help you.