diff --git a/Milter/dsn.py b/Milter/dsn.py
index fc314c5..961858e 100644
--- a/Milter/dsn.py
+++ b/Milter/dsn.py
@@ -95,7 +95,10 @@ Received-SPF: %(spf_result)s
 """
 
 def send_dsn(mailfrom,receiver,msg=None):
-  "Send DSN.  If msg is None, do callback verification."
+  """Send DSN.  If msg is None, do callback verification.
+     Mailfrom is original sender we are sending DSN or CBV to.
+     Receiver is the MTA sending the DSN.
+     Return None for success or (code,msg) for failure."""
   user,domain = mailfrom.split('@')
   q = spf.query(None,None,None)
   mxlist = q.dns(domain,'MX')
@@ -112,7 +115,7 @@ def send_dsn(mailfrom,receiver,msg=None):
       if resp.split()[0] == receiver:
         return (553,'Fraudulent MX for %s' % domain)
       if not (200 <= code <= 299):
-	raise SMTPHeloError(code, resp)
+	raise smtplib.SMTPHeloError(code, resp)
       if msg:
         try:
 	  smtp.sendmail('<>',mailfrom,msg)
@@ -122,7 +125,7 @@ def send_dsn(mailfrom,receiver,msg=None):
       else:	# CBV
 	code,resp = smtp.docmd('MAIL FROM: <>')
 	if code != 250:
-	  raise SMTPSenderRefused(code, resp, '<>')
+	  raise smtplib.SMTPSenderRefused(code, resp, '<>')
 	code,resp = smtp.rcpt(mailfrom)
 	if code not in (250,251):
 	  return (code,resp)		# permanent error
@@ -131,9 +134,9 @@ def send_dsn(mailfrom,receiver,msg=None):
     except smtplib.SMTPRecipientsRefused,x:
       return x.recipients[mailfrom]	# permanent error
     except smtplib.SMTPSenderRefused,x:
-      return x		# does not accept DSN
+      return x.args[:2]			# does not accept DSN
     except smtplib.SMTPDataError,x:
-      return x				# permanent error
+      return x.args			# permanent error
     except smtplib.SMTPException:
       pass		# any other error, try next MX
     except socket.error: