I've done some test with eximstats with those two options.
With '-byhost', eximstats takes the full name found in the 'H=x.y.z' entry in
the log and with '-bydomain' eximstats takes the name found the in the
'H=x.y.z' entry but removes the 'x.' to compute the league tables.
But I expect that with -bydomain, eximstats takes the information that follow
the '@' in the email address of the sender or the recepient to compute the
league tables.
