diff -urN ../exim-4.89.orig/src/transports/smtp.c ./src/transports/smtp.c
--- ../exim-4.89.orig/src/transports/smtp.c	2017-03-04 23:21:35.000000000 +0200
+++ ./src/transports/smtp.c	2017-03-10 13:13:00.000000000 +0200
@@ -849,6 +849,13 @@
     if (!sx->verify)
       msglog_line(sx->host, addr->message);
 
+    /* Stupid postfix answers 4xx for unknown users */
+    if ((sx->buffer[0] == '4') && (Ustrstr(sx->buffer, "Recipient address rejected: User unknown in local recipient table"))) sx->buffer[0] = '5';
+    if ((sx->buffer[0] == '4') && (Ustrstr(sx->buffer, "Recipient address rejected: User unknown in virtual mailbox table"))) sx->buffer[0] = '5';
+    if ((sx->buffer[0] == '4') && (Ustrstr(sx->buffer, "Recipient address rejected: User unknown in relay recipient table"))) sx->buffer[0] = '5';
+    if ((sx->buffer[0] == '4') && (Ustrstr(sx->buffer, "unknown user account"))) sx->buffer[0] = '5';
+    if ((sx->buffer[0] == '4') && (Ustrstr(sx->buffer, "Recipient address rejected: undeliverable address"))) sx->buffer[0] = '5';
+
     /* The response was 5xx */
 
     if (sx->buffer[0] == '5')
diff -urN ../exim-4.89.orig/src/verify.c ./src/verify.c
--- ../exim-4.89.orig/src/verify.c	2017-03-04 23:21:35.000000000 +0200
+++ ./src/verify.c	2017-03-10 13:10:45.000000000 +0200
@@ -446,6 +446,13 @@
 	    addr->user_message =
 	      string_sprintf("Callout verification failed:\n%s", resp);
 
+            /* Stupid postfix answers 4xx for unknown users */
+            if ((resp[0] == '4') && (Ustrstr(resp, "Recipient address rejected: User unknown in local recipient table"))) resp[0] = '5';
+            if ((resp[0] == '4') && (Ustrstr(resp, "Recipient address rejected: User unknown in virtual mailbox table"))) resp[0] = '5';
+            if ((resp[0] == '4') && (Ustrstr(resp, "Recipient address rejected: User unknown in relay recipient table"))) resp[0] = '5';
+            if ((resp[0] == '4') && (Ustrstr(resp, "unknown user account"))) resp[0] = '5';
+            if ((resp[0] == '4') && (Ustrstr(resp, "Recipient address rejected: undeliverable address"))) resp[0] = '5';
+
 	    /* Hard rejection ends the process */
 
 	    if (resp[0] == '5')   /* Address rejected */