Index: Lib/smtplib.py
===================================================================
--- Lib/smtplib.py	(revision 71798)
+++ Lib/smtplib.py	(working copy)
@@ -452,6 +452,7 @@
         optionlist = ''
         if options and self.does_esmtp:
             optionlist = ' ' + ' '.join(options)
+        sender = sender.decode()
         self.putcmd("mail", "FROM:%s%s" % (quoteaddr(sender) ,optionlist))
         return self.getreply()
 
@@ -545,7 +546,8 @@
             return encode_base64(response)
 
         def encode_plain(user, password):
-            return encode_base64("\0%s\0%s" % (user, password))
+            b0 = b"\0"
+            return encode_base64(b0 + b0.join(user + password))
 
 
         AUTH_PLAIN = "PLAIN"
@@ -580,7 +582,7 @@
             (code, resp) = self.docmd(encode_cram_md5(resp, user, password))
         elif authmethod == AUTH_PLAIN:
             (code, resp) = self.docmd("AUTH",
-                AUTH_PLAIN + " " + encode_plain(user, password))
+                AUTH_PLAIN + " " + encode_plain(user, password)[:-1])
         elif authmethod == AUTH_LOGIN:
             (code, resp) = self.docmd("AUTH",
                 "%s %s" % (AUTH_LOGIN, encode_base64(user)))