If the server rejects an email address, ask again rather than erroring

This is essentially symmetrical with cases where the client itself can tell
that what the user entered isn't an email address.

letsencrypt/client.py

@@ -115,7 +115,7 @@ def register(config, account_storage, tos_cb=None):
             backend=default_backend())))
     acme = acme_from_config_key(config, key)
     # TODO: add phone?
-    regr = acme.register(messages.NewRegistration.from_data(email=config.email))
+    regr = perform_registration(acme, config)
 
     if regr.terms_of_service is not None:
         if tos_cb is not None and not tos_cb(regr):
@@ -130,6 +130,21 @@ def register(config, account_storage, tos_cb=None):
 
     return acc, acme
 
+def perform_registration(acme, config):
+    """
+    Actually register new account, trying repeatedly if there are email
+    problems
+
+    :returns: the same value as acme.register
+    """
+    try:
+        regr = acme.register(messages.NewRegistration.from_data(email=config.email))
+    except messages.Error, e:
+        if "MX record" in repr(e):
+            config.namespace.email = display_ops.get_email(more=True, invalid=True)
+            return perform_registration(acme, config)
+        else:
+            raise
 
 class Client(object):
     """ACME protocol client.