a thread level representing a response level of {HEAD ~ each} of {TAIL ~ the at least two e mail messages} within the mail thread and inserted into a thread level field; and 
of
a thread level representing a response level of { HEAD ~ each } of { TAIL ~ the at least two e mail messages } within the mail thread and inserted into a thread level field ; and
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH