The cache is basically exactly the same as the browser cache. Thunderbird can display RSS feeds, which are basically web sites. Web content can be cached.

Letting CCleaner mess with files in the Thunderbird profile usually is a bad idea. If you feel you need to clean the cache, do that via Thunderbird Options. It is recommended to uncheck all CCleaner options for Thunderbird. The same goes for Firefox in case you use it.

Thanks for the help with that... I was surprised that T-Bird caches internet data when it seemingly does not access the net very much.