diff -ruNp sendmail-8.15.2.orig/sendmail/envelope.c sendmail-8.15.2/sendmail/envelope.c
--- sendmail-8.15.2.orig/sendmail/envelope.c	2014-06-12 19:30:47.000000000 +0200
+++ sendmail-8.15.2/sendmail/envelope.c	2018-02-21 12:59:15.364725322 +0100
@@ -323,7 +323,7 @@ dropenvelope(e, fulldrop, split)
 
 			/* don't free, allocated from e_rpool */
 			e->e_message = sm_rpool_strdup_x(e->e_rpool, buf);
-			message(buf);
+			message("%s", buf);
 			e->e_flags |= EF_CLRQUEUE;
 		}
 		if (msg_timeout == MSG_NOT_BY)
@@ -420,7 +420,7 @@ dropenvelope(e, fulldrop, split)
 				/* don't free, allocated from e_rpool */
 				e->e_message = sm_rpool_strdup_x(e->e_rpool,
 								 buf);
-				message(buf);
+				message("%s", buf);
 				e->e_flags |= EF_WARNING;
 			}
 			if (msg_timeout == MSG_WARN_BY)
diff -ruNp sendmail-8.15.2.orig/sendmail/parseaddr.c sendmail-8.15.2/sendmail/parseaddr.c
--- sendmail-8.15.2.orig/sendmail/parseaddr.c	2015-03-18 12:47:12.000000000 +0100
+++ sendmail-8.15.2/sendmail/parseaddr.c	2018-02-21 12:59:15.386725225 +0100
@@ -218,7 +218,7 @@ parseaddr(addr, a, flags, delim, delimpt
 			msg = "Deferring message until queue run";
 		if (tTd(20, 1))
 			sm_dprintf("parseaddr: queueing message\n");
-		message(msg);
+		message("%s", msg);
 		if (e->e_message == NULL && e->e_sendmode != SM_DEFER)
 			e->e_message = sm_rpool_strdup_x(e->e_rpool, msg);
 		a->q_state = QS_QUEUEUP;
diff -ruNp sendmail-8.15.2.orig/sendmail/srvrsmtp.c sendmail-8.15.2/sendmail/srvrsmtp.c
--- sendmail-8.15.2.orig/sendmail/srvrsmtp.c	2015-03-18 12:47:12.000000000 +0100
+++ sendmail-8.15.2/sendmail/srvrsmtp.c	2018-02-21 12:59:15.409725123 +0100
@@ -122,6 +122,26 @@ extern ENVELOPE	BlankEnvelope;
 #define SKIP_SPACE(s)	while (isascii(*s) && isspace(*s))	\
 				(s)++
 
+static inline void
+message1(fmt)
+	char *fmt;
+{
+	if (strchr(fmt, '%') == NULL)
+		message(fmt, NULL);
+	else
+		message("%s", fmt);
+}
+
+static inline void
+usrerr1(fmt)
+	char *fmt;
+{
+	if (strchr(fmt, '%') == NULL)
+		usrerr(fmt, NULL);
+	else
+		usrerr("%s", fmt);
+}
+
 /*
 **  PARSE_ESMTP_ARGS -- parse EMSTP arguments (for MAIL, RCPT)
 **
@@ -578,13 +598,13 @@ static bool	smtp_data __P((SMTP_T *, ENV
 				bool tsave = QuickAbort;		\
 									\
 				QuickAbort = false;			\
-				usrerr(response);			\
+				usrerr1(response);			\
 				QuickAbort = tsave;			\
 				e->e_sendqueue = NULL;			\
 				goto doquit;				\
 			}						\
 			else						\
-				usrerr(response);			\
+				usrerr1(response);			\
 			break;						\
 									\
 		  case SMFIR_REJECT:					\
@@ -931,7 +951,7 @@ smtp(nullserver, d_flags, e)
 	}
 	else if (strncmp(nullserver, "421 ", 4) == 0)
 	{
-		message(nullserver);
+		message1(nullserver);
 		goto doquit;
 	}
 
@@ -1849,7 +1869,7 @@ smtp(nullserver, d_flags, e)
 				if (nullserver != NULL)
 				{
 					if (ISSMTPREPLY(nullserver))
-						usrerr(nullserver);
+						usrerr1(nullserver);
 					else
 						usrerr("550 5.0.0 %s",
 						       nullserver);
@@ -2452,7 +2472,7 @@ smtp(nullserver, d_flags, e)
 					tempfail = true;
 					smtp.sm_milterize = false;
 					if (response != NULL)
-						usrerr(response);
+						usrerr1(response);
 					else
 						message("421 4.7.0 %s closing connection",
 							MyHostName);
@@ -3659,7 +3679,7 @@ smtp_data(smtp, e)
 				(void) extenhsc(response + 4, ' ', e->e_enhsc);
 #endif /* _FFR_MILTER_ENHSC */
 
-			usrerr(response);
+			usrerr1(response);
 			if (strncmp(response, "421 ", 4) == 0
 			    || strncmp(response, "421-", 4) == 0)
 			{
@@ -3779,7 +3799,7 @@ smtp_data(smtp, e)
 			if (ISSMTPCODE(response))
 				(void) extenhsc(response + 4, ' ', e->e_enhsc);
 #endif /* _FFR_MILTER_ENHSC */
-			usrerr(response);
+			usrerr1(response);
 			if (strncmp(response, "421 ", 4) == 0
 			    || strncmp(response, "421-", 4) == 0)
 				rv = false;