smtpd: broken sendmail compatibility

This has been a defect since OpenBSD 5.6 when OpenSMTPD was made the
default MTA.

When invoked as sendmail(8), smtpctl(8) implements a minimal set of
features from Sendmail. As per the man page:

  To maintain compatibility with Sendmail, Inc.'s implementation of
  sendmail, various other flags are accepted, but have no effect.
  
This is generally ok, EXCEPT when sendmail is invoked with -bs.

The -bs flag is defined as follows:

  Use the SMTP protocol as described in RFC821 on standard input
  and output.

This implies a very different mode of operation and will cause some
applications to hang if they invoke `sendmail -bs` because they
expect a RFC821 response on stdio. The way it behaves is broken.

Since sendmail(8) does not implement this function, it shall reject
the -bs flag if invoked to allow the calling application to error out.

I've posted this patch before a few times but never received feedback.

Regards
Lloyd

Index: usr.sbin/smtpd/smtpctl.c
===================================================================
RCS file: /cvs/src/usr.sbin/smtpd/smtpctl.c,v
diff -u -p -u -r1.178 smtpctl.c
--- usr.sbin/smtpd/smtpctl.c	3 Aug 2026 06:58:55 -0000	1.178
+++ usr.sbin/smtpd/smtpctl.c	15 Aug 2026 00:24:21 -0000
@@ -1115,10 +1115,16 @@ sendmail_compat(int argc, char **argv)
 		/*
 		 * determine whether we are called with flags
 		 * that should invoke makemap/newaliases.
+		 *
+		 * if unsupported sendmail -bs mode is invoked,
+		 * return an error.  
 		 */
-		for (i = 1; i < argc; i++)
+		for (i = 1; i < argc; i++) {
 			if (strncmp(argv[i], "-bi", 3) == 0)
 				exit(makemap(P_SENDMAIL, argc, argv));
+			if (strncmp(argv[i], "-bs", 3) == 0)
+				errx(1, "-bs mode is not supported");
+		}
 
 		if (!srv_connect())
 			offlinefp = offline_file();