diff -urN src.orig/transports/smtp.c src/transports/smtp.c
--- src.orig/transports/smtp.c	Fri Sep 28 15:21:57 2007
+++ src/transports/smtp.c	Thu May  1 17:31:21 2008
@@ -728,6 +728,13 @@
     setflag(addr, af_pass_message);
     deliver_msglog("%s %s\n", tod_stamp(tod_log), addr->message);
 
+    /* Stupid postfix answers 4xx for unknown users */
+    if ((buffer[0] == '4') && (Ustrstr(buffer, "Recipient address rejected: User unknown in local recipient table"))) buffer[0] = '5';
+    if ((buffer[0] == '4') && (Ustrstr(buffer, "Recipient address rejected: User unknown in virtual mailbox table"))) buffer[0] = '5';
+    if ((buffer[0] == '4') && (Ustrstr(buffer, "Recipient address rejected: User unknown in relay recipient table"))) buffer[0] = '5';
+    if ((buffer[0] == '4') && (Ustrstr(buffer, "unknown user account"))) buffer[0] = '5';
+    if ((buffer[0] == '4') && (Ustrstr(buffer, "Recipient address rejected: undeliverable address"))) buffer[0] = '5';
+
     /* The response was 5xx */
 
     if (buffer[0] == '5')
diff -urN src.orig/verify.c src/verify.c
--- src.orig/verify.c	Thu Jun 14 17:18:19 2007
+++ src/verify.c	Thu May  1 17:32:08 2008
@@ -722,6 +722,13 @@
         string_sprintf("Called:   %s\nSent:     %s\nResponse: %s",
           host->address, big_buffer, responsebuffer);
 
+      /* Stupid postfix answers 4xx for unknown users */
+      if ((responsebuffer[0] == '4') && (Ustrstr(responsebuffer, "Recipient address rejected: User unknown in local recipient table"))) responsebuffer[0] = '5';
+      if ((responsebuffer[0] == '4') && (Ustrstr(responsebuffer, "Recipient address rejected: User unknown in virtual mailbox table"))) responsebuffer[0] = '5';
+      if ((responsebuffer[0] == '4') && (Ustrstr(responsebuffer, "Recipient address rejected: User unknown in relay recipient table"))) responsebuffer[0] = '5';
+      if ((responsebuffer[0] == '4') && (Ustrstr(responsebuffer, "unknown user account"))) responsebuffer[0] = '5';
+      if ((responsebuffer[0] == '4') && (Ustrstr(responsebuffer, "Recipient address rejected: undeliverable address"))) responsebuffer[0] = '5';
+
       /* Hard rejection ends the process */
 
       if (responsebuffer[0] == '5')   /* Address rejected */