Issue #23681: Fixed Python 2 to 3 poring bugs.
Indexing bytes retiurns an integer, not bytes.
diff --git a/Lib/smtpd.py b/Lib/smtpd.py
index 33653d4..0fae170 100755
--- a/Lib/smtpd.py
+++ b/Lib/smtpd.py
@@ -154,7 +154,7 @@
         else:
             self._emptystring = b''
             self._linesep = b'\r\n'
-            self._dotsep = b'.'
+            self._dotsep = ord(b'.')
             self._newline = b'\n'
         self._set_rset_state()
         self.seen_greeting = ''