Only in netqmail-1.05-logging/: logging-4.patch
diff -burp netqmail-1.05-orig/qmail-smtpd.c netqmail-1.05-logging/qmail-smtpd.c
--- netqmail-1.05-orig/qmail-smtpd.c	2010-09-28 10:22:37.489770109 -0400
+++ netqmail-1.05-logging/qmail-smtpd.c	2018-08-22 22:13:47.971312525 -0400
@@ -38,16 +38,20 @@ int safewrite(fd,buf,len) int fd; char *
 
 char ssoutbuf[512];
 substdio ssout = SUBSTDIO_FDBUF(safewrite,1,ssoutbuf,sizeof ssoutbuf);
+char sserrbuf[512];
+substdio sserr = SUBSTDIO_FDBUF(write,2,sserrbuf,sizeof sserrbuf);
 
+void logit(const char* message);
+void logit2(const char* message, const char* reason);
 void flush() { substdio_flush(&ssout); }
 void out(s) char *s; { substdio_puts(&ssout,s); }
 
-void die_read() { _exit(1); }
-void die_alarm() { out("451 timeout (#4.4.2)\r\n"); flush(); _exit(1); }
+void die_read() { logit("read failed"); _exit(1); }
+void die_alarm() { logit("timeout"); out("451 timeout (#4.4.2)\r\n"); flush(); _exit(1); }
 void die_nomem() { out("421 out of memory (#4.3.0)\r\n"); flush(); _exit(1); }
-void die_control() { out("421 unable to read controls (#4.3.0)\r\n"); flush(); _exit(1); }
-void die_ipme() { out("421 unable to figure out my IP addresses (#4.3.0)\r\n"); flush(); _exit(1); }
-void straynewline() { out("451 See http://pobox.com/~djb/docs/smtplf.html.\r\n"); flush(); _exit(1); }
+void die_control() { logit("unable to read controls"); out("421 unable to read controls (#4.3.0)\r\n"); flush(); _exit(1); }
+void die_ipme() { logit("unable to figure out my IP addresses"); out("421 unable to figure out my IP addresses (#4.3.0)\r\n"); flush(); _exit(1); }
+void straynewline() { logit("bad newlines"); out("451 See http://pobox.com/~djb/docs/smtplf.html.\r\n"); flush(); _exit(1); }
 
 void err_bmf() { out("553 sorry, your envelope sender is in my badmailfrom list (#5.7.1)\r\n"); }
 void err_nogateway() { out("553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)\r\n"); }
@@ -221,6 +225,48 @@ int seenmail = 0;
 int flagbarf; /* defined if seenmail */
 stralloc mailfrom = {0};
 stralloc rcptto = {0};
+stralloc log_buf = {0};
+
+void safeloglen(const char* string, const int len) {
+    if (string && len) {
+        if (!stralloc_catb(&log_buf, string, len-1)) die_nomem();
+    } else {
+        if (!stralloc_catb(&log_buf, "(null)", 6)) die_nomem();
+    }
+}
+
+void safelog(const char* string) {
+    if (string) {
+        if (!stralloc_cats(&log_buf, string)) die_nomem();
+    } else {
+        if (!stralloc_catb(&log_buf, "(null)", 6)) die_nomem();
+    }
+}
+
+void logit(const char* message) {
+    logit2(message, (const char*)0);
+}
+
+void logit2(const char* message, const char* reason)
+{
+  if (!stralloc_copys(&log_buf, "qmail-smtpd: ")) die_nomem();
+  safelog(message);
+  if (reason) {
+      if (!stralloc_cats(&log_buf, " (")) die_nomem();
+      if (!stralloc_cats(&log_buf, reason)) die_nomem();
+      if (!stralloc_cats(&log_buf, "): ")) die_nomem();
+  }
+  if (!stralloc_catb(&log_buf, ": ", 2)) die_nomem();
+  safeloglen(mailfrom.s, mailfrom.len);
+  if (!stralloc_catb(&log_buf, " from ", 6)) die_nomem();
+  safelog(remoteip);
+  if (!stralloc_catb(&log_buf, " to ", 4)) die_nomem();
+  safeloglen(addr.s, addr.len);
+  if (!stralloc_catb(&log_buf, " helo ", 6)) die_nomem();
+  safeloglen(helohost.s, helohost.len);
+  if (!stralloc_catb(&log_buf, "\n", 1)) die_nomem();
+  substdio_putflush(&sserr, log_buf);
+}
 
 void smtp_helo(arg) char *arg;
 {
@@ -250,7 +296,7 @@ void smtp_mail(arg) char *arg;
 void smtp_rcpt(arg) char *arg; {
   if (!seenmail) { err_wantmail(); return; }
   if (!addrparse(arg)) { err_syntax(); return; }
-  if (flagbarf) { err_bmf(); return; }
+  if (flagbarf) { logit("badmailfrom"); err_bmf(); return; }
   if (relayclient) {
     --addr.len;
     if (!stralloc_cats(&addr,relayclient)) die_nomem();
@@ -386,10 +432,24 @@ void smtp_data(arg) char *arg; {
   qmail_put(&qqt,rcptto.s,rcptto.len);
  
   qqx = qmail_close(&qqt);
-  if (!*qqx) { acceptmessage(qp); return; }
-  if (hops) { out("554 too many hops, this message is looping (#5.4.6)\r\n"); return; }
-  if (databytes) if (!bytestooverflow) { out("552 sorry, that message size exceeds my databytes limit (#5.3.4)\r\n"); return; }
-  if (*qqx == 'D') out("554 "); else out("451 ");
+  if (!*qqx) { acceptmessage(qp); logit("message accepted"); return; }
+  if (hops) {
+    out("554 too many hops, this message is looping (#5.4.6)\r\n");
+    logit("message looping");
+    return;
+  }
+  if (databytes) if (!bytestooverflow) {
+    out("552 sorry, that message size exceeds my databytes limit (#5.3.4)\r\n");
+    logit("message too big");
+    return;
+  }
+  if (*qqx == 'D') {
+    out("554 ");
+    logit2("message rejected", qqx + 1);
+  } else {
+    out("451 ");
+    logit2("message delayed", qqx + 1);
+  }
   out(qqx + 1);
   out("\r\n");
 }