When encode is not possible, assume it's already bytes, so return it raw with a warning

envelope.py

@@ -44,7 +44,11 @@ def assume_rfc3156(msg):
 
 def assume_oldstyle(payload):
     yield MailTest("Found BEGIN PGP SIGNED MESSAGE: message treated as old style PGP email.")
-    yield payload.encode()
+    try:
+        yield payload.encode()
+    except:
+        yield MailTest("Non-armored signed message discovered. Avoid using binary message over SMTP (see RFC2015 #2. PGP data formats).", 2)
+        yield payload