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

.ip \fBnext\fP
The
.b next
command goes to the next message and types it.  If given a message list,
.b next
goes to the first such message and types it.  Thus,
.(l
next root
.)l
goes to the next message sent by
.q root
and types it.  The
.b next
command can be abbreviated to simply a newline, which means that one
can go to and type a message by simply giving its message number or
one of the magic characters
.q "\(ua"
.q "."
or
.q "$".
Thus,
.(l
\&.
.)l
prints the current message and
.(l
4
.)l
prints message 4, as described previously.