diff -urN ../exim-4.88.orig/src/transports/smtp.c ./src/transports/smtp.c
--- ../exim-4.88.orig/src/transports/smtp.c	2016-12-18 16:02:28.000000000 +0200
+++ ./src/transports/smtp.c	2016-12-29 22:50:36.853431154 +0200
@@ -886,6 +886,13 @@
     setflag(addr, af_pass_message);
     msglog_line(host, 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 ../exim-4.88.orig/src/verify.c ./src/verify.c
--- ../exim-4.88.orig/src/verify.c	2016-12-18 16:02:28.000000000 +0200
+++ ./src/verify.c	2016-12-29 22:49:34.835432591 +0200
@@ -1253,6 +1253,13 @@
           : 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 */