diff --git a/RewriteGALAddressesToSMTP.py b/RewriteGALAddressesToSMTP.py
index 66c74b9..2013617 100644
--- a/RewriteGALAddressesToSMTP.py
+++ b/RewriteGALAddressesToSMTP.py
@@ -1,5 +1,6 @@
 # kopano-dagent RewriteGALAddressesToSMTP
 # (c) 2017 Hein-Pieter van Braam <hp@tmm.cx>
+# (C) 2020 Christoph Haas <christoph.h@sprinternet.at>
 # Licensed under the GPU Affero GPL v3 (or at your option any later version)
 # See LICENSE file for details
 
@@ -16,37 +17,75 @@ class RewriteGALAddressesToSMTP(IMapiDAgentPlugin):
     def __init__(self, logger):
         IMapiDAgentPlugin.__init__(self, logger)
 
-    def DecodeHeaderToUTF8(self, header):
-        decodedHeader = decode_header(header)
-        headerText, headerEncoding = decodedHeader[0]
-        if headerEncoding is None:
-            return unicode(headerText)
-        else:
-            return unicode(headerText, headerEncoding, 'ignore')
+    def DecodeHeaderToUTF8(self, header) -> str:
+        decodedHeaderParts = decode_header(header)
 
+        strHeader = ""
+        for headerText, headerEncoding in decodedHeaderParts:
+            if headerEncoding is None:
+                if isinstance(headerText, str):
+                    strHeader += headerText  # already a string
+                else:
+                    strHeader += headerText.decode('ASCII')
+            else:
+                strHeader += headerText.decode(headerEncoding)
+        return strHeader
+
+    def GetKopanoRecipient(self, emailAddr: bytes, kopanoRecipients: list) -> list:
+        for rcpt in kopanoRecipients:
+            if SPropValue(PR_EMAIL_ADDRESS, emailAddr) in rcpt:
+                return rcpt
+        return None
+
+    # PreDelivery hook entry point
     def PreDelivery(self, session, addrbook, store, folder, message):
+        # Load headers and extract To: and CC: headers
         headers = message.GetProps([PR_TRANSPORT_MESSAGE_HEADERS], 0)[0].Value
-        msg = email.message_from_string(headers)
+        msg = email.message_from_bytes(headers)
         to_addrs = getaddresses(msg.get_all('to', []))
         cc_addrs = getaddresses(msg.get_all('cc', []))
-    
+
+        # First load all original recipients for the message
+        orig_to_addrs = []
+        orig_cc_addrs = []
+        rcptTable = message.GetRecipientTable(0)
+        #rcptTable.SetColumns([PR_EMAIL_ADDRESS, PR_ADDRTYPE], 0)  # no specific columns means: load all columns
+        origRcpts = rcptTable.QueryRows(10, 0)
+        while origRcpts and len(origRcpts) > 0:
+            for origRcpt in origRcpts:
+                if SPropValue(PR_RECIPIENT_TYPE, MAPI_TO) in origRcpt:
+                    orig_to_addrs.append(origRcpt)
+                if SPropValue(PR_RECIPIENT_TYPE, MAPI_CC) in origRcpt:
+                    orig_cc_addrs.append(origRcpt)
+            origRcpts = rcptTable.QueryRows(10, 0)  # Read the next 10 recipients
+
+
+        # Now replace alias addresses with the SMTP address type
         names = []
         for addr in to_addrs:
-            names.append([
-                SPropValue(PR_RECIPIENT_TYPE, MAPI_TO),
-                SPropValue(PR_DISPLAY_NAME_W, self.DecodeHeaderToUTF8(addr[0])),
-                SPropValue(PR_ADDRTYPE, 'SMTP'),
-                SPropValue(PR_EMAIL_ADDRESS, unicode(addr[1])),
-            ])
-    
-        for addr in cc_addrs:
-            names.append([
-                SPropValue(PR_RECIPIENT_TYPE, MAPI_CC),
-                SPropValue(PR_DISPLAY_NAME_W, self.DecodeHeaderToUTF8(addr[0])),
-                SPropValue(PR_ADDRTYPE, 'SMTP'),
-                SPropValue(PR_EMAIL_ADDRESS, unicode(addr[1])),
-            ])
-    
-        message.ModifyRecipients(0, names)
-        return MP_CONTINUE,
+            kopanoRcpt = self.GetKopanoRecipient(bytes(addr[1], encoding='utf8'), orig_to_addrs)
+            if kopanoRcpt is not None:
+                names.append(kopanoRcpt)  # keep original To: entry
+            else:
+                names.append([
+                    SPropValue(PR_RECIPIENT_TYPE, MAPI_TO),
+                    SPropValue(PR_DISPLAY_NAME, bytes(self.DecodeHeaderToUTF8(addr[0]), encoding="utf8")),
+                    SPropValue(PR_ADDRTYPE, bytes('SMTP', encoding="utf8")),
+                    SPropValue(PR_EMAIL_ADDRESS, bytes(addr[1], encoding='utf8')),
+                ])
 
+        for addr in cc_addrs:
+            kopanoRcpt = self.GetKopanoRecipient(bytes(addr[1], encoding='utf8'), orig_cc_addrs)
+            if kopanoRcpt is not None:
+                names.append(kopanoRcpt)  # keep original CC: entry
+            else:
+                names.append([
+                    SPropValue(PR_RECIPIENT_TYPE, MAPI_CC),
+                    SPropValue(PR_DISPLAY_NAME, bytes(self.DecodeHeaderToUTF8(addr[0]), encoding="utf8")),
+                    SPropValue(PR_ADDRTYPE, bytes('SMTP', encoding="utf8")),
+                    SPropValue(PR_EMAIL_ADDRESS, bytes(addr[1], encoding='utf8')),
+                ])
+
+        if len(names) > 0:  # double check, avoid empty recipients
+            message.ModifyRecipients(0, names)
+        return MP_CONTINUE,
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