:0
* ^X-Trained-As: \/(h|sp)am
{ TRAINED_AS=$MATCH }

# remove all headers our mailfilter adds
:0 fw
|$FORMAIL -IX-EqDomain -IX-EqTo -IX-Justme \
          -IX-Trained-As \
          -IX-UID -IX-OfflineIMAP- \
          -IX-Status -IStatus -IX-Keywords -IContent-Length

# now should be idempotent