It would be great to know, what filter on an e-mail was applied.
Therefore, every created filter becomes an unique number.
Beside the shield icon (for trackers etc.) in the mail would be enough place for the filter icon :).
If there was no filter applied, the icon turns green without number.
Was there an allow-filter, the icon turns green with the number of the filter.
A block-filter turns the icon red with the number of the block filter, that's all.