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

# remove all headers added in the *local* mail chain
:0 fw
|$FORMAIL \
        # headers added by this mailfilter
          -IX-EqDomain -IX-EqTo -IX-Justme \
        # headers added by the retraining system
          -IX-Trained-As \
        # headers added by offlineimap
          -IX-UID -IX-OfflineIMAP- \
        # headers added by the MUA
          -IX-Status -IStatus -IX-Keywords -IContent-Length

# now should be idempotent