V8/usr/src/cmd/Mail/manual/cmds/top

.ip \fBtop\fP
The
.b top
command takes a message list and prints the first five lines
of each addressed message.  It may be abbreviated to
.b to .
If you wish, you can change the number of lines that
.b top
prints out by setting the valued option
.q "toplines."
On a CRT terminal,
.(l
set toplines=10
.)l
might be preferred.