Adding missing information:
E-mail messages are intended to be brief,
and the author must trim out peripheral detail.
This involves choices about what information is and is not critical
to the topic at hand.
If the author does not mention an item that you regard as relevant,
do not assume that he is ignorant those facts.
For example, if someone sends a message about an act of vandalism
and asking anyone observed relevant suspicious activity,
it is grossly inappropriate to send a reply to the list upbraiding
the sender for not knowing and/or detailing the historical importance
of the item vandalized -- its historical value likely has no relevance
to whether someone saw anything suspicious.
Just as in other discussions, new information and new considerations
can be added to an e-mail discussion
without denigrating the other participants.