diff -urNp -x '*.orig' sendmail-8.16.1.org/sendmail/collect.c sendmail-8.16.1/sendmail/collect.c
--- sendmail-8.16.1.org/sendmail/collect.c	2020-05-19 21:54:33.000000000 +0200
+++ sendmail-8.16.1/sendmail/collect.c	2021-10-03 22:30:58.038924399 +0200
@@ -15,6 +15,8 @@
 
 #include <sm/sendmail.h>
 
+#define MAXHDRZ 512
+
 static void	eatfrom __P((char *volatile, ENVELOPE *));
 static void	collect_doheader __P((ENVELOPE *));
 static SM_FILE_T *collect_dfopen __P((ENVELOPE *));
@@ -283,6 +285,7 @@ collect(fp, smtpmode, hdrp, e, rsetsize)
 	register ENVELOPE *e;
 	bool rsetsize;
 {
+	int hdrz = 0;
 	register SM_FILE_T *df;
 	bool ignrdot;
 	int dbto;
@@ -612,6 +615,16 @@ nextstate:
 				mstate = MS_BODY;
 				goto nextstate;
 			}
+ 
+            if (hdrz++>MAXHDRZ)
+                        {
+                                sm_syslog(LOG_NOTICE, e->e_id,
+                                "excessive headers from %s during message collect",
+                                CurHostName ? CurHostName : "<local machine>");
+                                errno = 0;
+                                usrerr("451 Stop this. You are lame.");
+                                goto readdone;
+                        }
 
 			/* check for possible continuation line */
 			do