+If nmh has been compiled with APOP #defined, the `\-apop' switch will cause
+\fImsgchk\fR to use APOP rather than standard POP3 authentication. Under APOP,
+a unique string (generally of the format
+<\fIpid\fR.\fItimestamp\fR@\fIhostname\fR>) is announced by the POP server.
+Rather than `USER \fIuser\fR', `PASS \fIpassword\fR', msgchk sends `APOP
+\fIuser\fR \fIdigest\fR', where digest is the MD5 hash of the unique string
+followed by a `secret' shared by client and server, essentially equivalent to
+the user's password (though an APOP-enabled POP3 server could have separate APOP
+and plain POP3 passwords for a single user). `\-noapop' disables APOP in cases
+where it'd otherwise be used.
+