Allow open realm signups with non-real-user MIT addresses.

This allows me to use a mail alias with CUSTOMER3, and closes #1671.

(imported from commit 581d1145fb112b9d35ad522fa06f81a25b4b8d3b)

zerver/forms.py

@@ -34,11 +34,13 @@ def not_mit_mailing_list(value):
         # Check whether the user exists and can get mail.
         try:
             DNS.dnslookup("%s.pobox.ns.athena.mit.edu" % username, DNS.Type.TXT)
+            return True
         except DNS.Base.ServerError, e:
             if e.rcode == DNS.Status.NXDOMAIN:
                 raise ValidationError(mark_safe(u'That user does not exist at MIT or is a <a href="https://ist.mit.edu/email-lists">mailing list</a>. If you want to sign up an alias for Zulip, <a href="mailto:support@zulip.com">contact us</a>.'))
             else:
                 raise
+    return True
 
 class RegistrationForm(forms.Form):
     full_name = forms.CharField(max_length=100)
@@ -56,7 +58,7 @@ class HomepageForm(forms.Form):
     if settings.ALLOW_REGISTER:
         email = forms.EmailField()
     else:
-        validators = [not_mit_mailing_list, is_inactive]
+        validators = [is_inactive]
         email = forms.EmailField(validators=validators)
 
     def __init__(self, *args, **kwargs):
@@ -67,7 +69,7 @@ class HomepageForm(forms.Form):
 
     def clean_email(self):
         data = self.cleaned_data['email']
-        if completely_open(self.domain) or has_valid_realm(data):
+        if completely_open(self.domain) or has_valid_realm(data) and not_mit_mailing_list(data):
             return data
         raise ValidationError(mark_safe(
                 u'Registration is not currently available for your domain. ' \