I also should have mentioned in the commit message for
sortm(1) -check/-nocheck that Norm proposed their addition.
.B \-check
switch,
.B sortm
-inhibits all modifications to the folder if there any such messages,
-and exits with non-zero status. With the default of
+inhibits all modifications to the folder if there are any such
+messages, and exits with non-zero status. With the default of
.BR \-nocheck ,
.B sortm
sorts messages with a missing or invalid