After the switch from Sendmail to smtpd in OpenBSD 5.6, applications using
`sendmail -bs` began to hang as this is handled inappropriately by smtpd.

The definition of -bs in Sendmail parlance:

> Use the SMTP protocol as described in RFC821 on standard input and output.
> This flag implies all the operations of the -ba flag that are compatible
> with SMTP.

Applications calling sendmail -bs will wait on stdin for an SMTP greeting:

220 foo.bar ESMTP Sendmail 8.14.8/8.14.8/Submit; Wed, 30 Apr 2025 19:03:40 GMT

Since smtpd does not support sendmail -bs mode, it shall explicitly reject
the flag and not simply ignore it, because -bs implies a very different
operating mode for the legacy Sendmail binary and any calling application.

Regards
Lloyd

Index: smtpctl.c
===================================================================
RCS file: /cvs/src/usr.sbin/smtpd/smtpctl.c,v
retrieving revision 1.176
diff -u -p -u -p -r1.176 smtpctl.c
--- smtpctl.c	21 Nov 2024 13:42:22 -0000	1.176
+++ smtpctl.c	3 Jan 2026 05:01:37 -0000
@@ -1115,10 +1115,15 @@ sendmail_compat(int argc, char **argv)
 		/*
 		 * determine whether we are called with flags
 		 * that should invoke makemap/newaliases.
+		 *
+		 * explicitly reject unsupported sendmail -bs mode
 		 */
-		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();