diff --git a/kallithea/lib/rcmail/smtp_mailer.py b/kallithea/lib/rcmail/smtp_mailer.py
--- a/kallithea/lib/rcmail/smtp_mailer.py
+++ b/kallithea/lib/rcmail/smtp_mailer.py
@@ -96,7 +96,7 @@ class SmtpMailer(object):
         if self.user and self.passwd:
             smtp_serv.login(self.user, self.passwd)
 
-        smtp_serv.sendmail(msg.sender, msg.send_to, raw_msg.as_string())
+        smtp_serv.sendmail(msg.sender, list(msg.send_to), raw_msg.as_string())
         logging.info('MAIL SENT TO: %s' % recipients)
 
         try:
diff --git a/kallithea/tests/other/test_mail.py b/kallithea/tests/other/test_mail.py
--- a/kallithea/tests/other/test_mail.py
+++ b/kallithea/tests/other/test_mail.py
@@ -21,9 +21,8 @@ class smtplib_mock(object):
 
     def sendmail(self, sender, dest, msg):
         smtplib_mock.lastsender = sender
-        smtplib_mock.lastdest = dest
+        smtplib_mock.lastdest = set(dest)
         smtplib_mock.lastmsg = msg
-        pass
 
 
 @mock.patch('kallithea.lib.rcmail.smtp_mailer.smtplib', smtplib_mock)