The response level may represent the relationship of the messages within {TAIL ~ the flow} of {HEAD ~ a conversation} in a mail thread 
The response level may represent the relationship of the messages within { TAIL ~ the flow } of { HEAD ~ a conversation } in a mail thread
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH