diff -Nur sendmail-8.12.0.orig/sendmail/conf.c sendmail-8.12.0/sendmail/conf.c
--- sendmail-8.12.0.orig/sendmail/conf.c	Wed Sep  5 00:43:02 2001
+++ sendmail-8.12.0/sendmail/conf.c	Mon Sep 24 10:01:52 2001
@@ -326,6 +326,8 @@
 	DontLockReadFiles = true;
 	DontProbeInterfaces = DPI_PROBEALL;
 	DoubleBounceAddr = "postmaster";
+	DetectTelnet = 0;
+	ExitOnTelnet = 1;
 	MaxHeadersLength = MAXHDRSLEN;
 	MaxForwardEntries = 0;
 	FastSplit = 1;
diff -Nur sendmail-8.12.0.orig/sendmail/err.c sendmail-8.12.0/sendmail/err.c
--- sendmail-8.12.0.orig/sendmail/err.c	Wed Sep  5 00:43:03 2001
+++ sendmail-8.12.0/sendmail/err.c	Mon Sep 24 10:02:57 2001
@@ -909,15 +909,17 @@
 		(void) sm_strlcpyn(eb, spaceleft, 2,
 				   shortenstring(to, MAXSHORTSTR), "... ");
 		spaceleft -= strlen(eb);
-		while (*eb != '\0')
-			*eb++ &= 0177;
+		if(!DetectTelnet)
+			while (*eb != '\0')
+				*eb++ &= 0177;
 	}
 
 	/* output the message */
 	(void) sm_vsnprintf(eb, spaceleft, fmt, ap);
 	spaceleft -= strlen(eb);
-	while (*eb != '\0')
-		*eb++ &= 0177;
+	if(!DetectTelnet)
+		while (*eb != '\0')
+			*eb++ &= 0177;
 
 	/* output the error code, if any */
 	if (eno != 0)
diff -Nur sendmail-8.12.0.orig/sendmail/readcf.c sendmail-8.12.0/sendmail/readcf.c
--- sendmail-8.12.0.orig/sendmail/readcf.c	Wed Sep  5 00:43:05 2001
+++ sendmail-8.12.0/sendmail/readcf.c	Mon Sep 24 10:06:43 2001
@@ -2039,6 +2039,10 @@
 # define O_SOFTBOUNCE	0xcf
 	{ "SoftBounce",	O_SOFTBOUNCE,	OI_NONE	},
 #endif /* _FFR_SOFT_BOUNCE */
+#define O_DETECTTELNET	0xd0
+	{ "DetectTelnet",	O_DETECTTELNET,		OI_NONE },
+#define O_EXITONTELNET	0xd1
+	{ "ExitOnTelnet",	O_EXITONTELNET,		OI_NONE },
 	{ NULL,				'\0',		OI_NONE	}
 };
 
@@ -3462,6 +3466,14 @@
 		break;
 #endif /* _FFR_SOFT_BOUNCE */
 
+	  case O_DETECTTELNET:		/* detect telnet */
+		DetectTelnet = atobool(val);
+		break;
+
+	  case O_EXITONTELNET:		/* exit if telnet detected */
+		ExitOnTelnet = atobool(val);
+		break;
+
 	  default:
 		if (tTd(37, 1))
 		{
diff -Nur sendmail-8.12.0.orig/sendmail/sendmail.h sendmail-8.12.0/sendmail/sendmail.h
--- sendmail-8.12.0.orig/sendmail/sendmail.h	Wed Sep  5 00:43:05 2001
+++ sendmail-8.12.0/sendmail/sendmail.h	Mon Sep 24 10:01:52 2001
@@ -2074,6 +2074,8 @@
 EXTERN bool	WorkAroundBrokenAAAA;	/* some nameservers return SERVFAIL on AAAA queries */
 EXTERN bool	UseErrorsTo;	/* use Errors-To: header (back compat) */
 EXTERN bool	UseNameServer;	/* using DNS -- interpret h_errno & MX RRs */
+EXTERN bool	DetectTelnet;	/* force telnet detection on/off */
+EXTERN bool	ExitOnTelnet;	/* exit if telnet detected */
 EXTERN char	InetMode;		/* default network for daemon mode */
 EXTERN char	OpMode;		/* operation mode, see below */
 EXTERN char	SpaceSub;	/* substitution for <lwsp> */
diff -Nur sendmail-8.12.0.orig/sendmail/srvrsmtp.c sendmail-8.12.0/sendmail/srvrsmtp.c
--- sendmail-8.12.0.orig/sendmail/srvrsmtp.c	Wed Sep  5 00:43:06 2001
+++ sendmail-8.12.0/sendmail/srvrsmtp.c	Mon Sep 24 10:09:38 2001
@@ -12,6 +12,7 @@
  */
 
 #include <sendmail.h>
+#include <arpa/telnet.h>
 #if MILTER
 # include <libmilter/mfdef.h>
 #endif /* MILTER */
@@ -369,6 +370,9 @@
 	char *args[MAXSMTPARGS];
 	char inp[MAXLINE];
 	char cmdbuf[MAXLINE];
+	char telnet_request[] = {IAC, DO, TELOPT_SGA, 0};
+	char telnet_response[] = {IAC, WILL, TELOPT_SGA, 0};
+	char *telnet_tmp;
 #if SASL
 	sasl_conn_t *conn;
 	volatile bool sasl_ok;
@@ -666,12 +669,8 @@
 	id = strchr(inp, ' ');
 	if (id == NULL)
 		id = &inp[strlen(inp)];
-	if (p == NULL)
-		(void) sm_snprintf(cmdbuf, sizeof cmdbuf,
-			 "%s %%.*s ESMTP%%s", greetcode);
-	else
-		(void) sm_snprintf(cmdbuf, sizeof cmdbuf,
-			 "%s-%%.*s ESMTP%%s", greetcode);
+	sm_snprintf(cmdbuf, sizeof cmdbuf, "%s%c%%.*s ESMTP%%s%s", greetcode,
+		(p == NULL) ? ' ' : '-', DetectTelnet ? telnet_request : "");
 	message(cmdbuf, (int) (id - inp), inp, id);
 
 	/* output remaining lines */
@@ -764,6 +763,20 @@
 			goto doquit;
 		}
 
+		if (DetectTelnet && strlen(inp) >= 3)
+			for (telnet_tmp = inp; *(telnet_tmp+2); telnet_tmp++)
+				if (!strncmp(telnet_tmp, telnet_response, 3))
+				{
+					sm_syslog(LOG_NOTICE, e->e_id,
+					  "telnet detected from %.100s (%.100s)",
+					  CurSmtpClient, anynet_ntoa(&RealHostAddr));
+					if (ExitOnTelnet)
+					{
+						message("571 Unauthorized usage prohibited");
+						goto doquit;
+					}
+				}
+
 #if _FFR_ADAPTIVE_EOL
 		if (first)
 		{