diff -r e76f1ee05cc8 courier/control.py
--- a/courier/control.py	Sun Apr 28 14:56:49 2019 -0700
+++ b/courier/control.py	Fri Aug 09 12:31:26 2019 +0200
@@ -73,7 +73,7 @@
     sender = get_senders_mta(control_paths)
     if not sender:
         return None
-    ipstr = sender.partition('[')[2].partition(']')[0]
+    ipstr = sender.rpartition('[')[2].partition(']')[0]
     if not ipstr:
         return None
     sender_ip = ipaddress.ip_address(ipstr)
diff -r e76f1ee05cc8 courier/xfilter.py
--- a/courier/xfilter.py	Sun Apr 28 14:56:49 2019 -0700
+++ b/courier/xfilter.py	Fri Aug 09 12:31:26 2019 +0200
@@ -26,6 +26,7 @@
     import email.generator
 import courier.control
 import courier.config
+import tempfile
 
 
 class XFilterError(Exception):
@@ -86,18 +87,24 @@
         return self.control_data
 
     def submit(self):
-        bfo = open(self.body_path, 'r+')
-        bfo.truncate(0)
-        g = email.generator.Generator(bfo, mangle_from_=False)
-        g.flatten(self.message)
-        # Make sure that the file ends with a newline, or courier
-        # will choke on the new message file.
-        bfo.seek(0, 2)
-        bfo.seek(bfo.tell() - 1, 0)
-        if bfo.read(1) != '\n':
-            bfo.seek(0, 2)
-            bfo.write('\n')
-        bfo.close()
+        with tempfile.TemporaryFile(mode='w+b', prefix='pythonfilter') as tmp:
+            g = email.generator.BytesGenerator(tmp, mangle_from_=False)
+            g.flatten(self.message)
+            # Make sure that the file ends with a newline, or courier
+            # will choke on the new message file.
+            tmp.seek(0, 2)
+            tmp.seek(tmp.tell() - 1, 0)
+            if tmp.read(1).decode('ascii') != '\n':
+                tmp.seek(0, 2)
+                tmp.write('\n'.encode('ascii'))
+            tmp.seek(0)
+            with open(self.body_path, 'r+b') as bfo:
+                bfo.truncate(0)
+                while True:
+                    data = tmp.read(4096)
+                    if not data:
+                        break
+                    bfo.write(data)
         return ''
 
     # Deprecated names preserved for compatibility with older releases