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

.ip \fBshell\fP
The
.b shell
command allows you to
escape to the shell.
.b Shell
invokes an interactive shell and allows you to type commands to it.
When you leave the shell, you will return to
.i Mail .
The shell used is a default assumed by
.i Mail ;
you can override this default by setting the valued option
.q SHELL,
eg:
.(l
set SHELL=/bin/csh
.)l