+.PP
+.B sortm
+always issues a warning for each message that is missing a
+\*(lqDate:\*(rq field, has a \*(lqDate:\*(rq field that cannot be
+parsed, or has a format error in any header field. With the
+.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
+.BR \-nocheck ,
+.B sortm
+sorts messages with a missing or invalid
+\*(lqDate:\*(rq field using their file modification times.