We already reply your support request email, but we always get error message :
This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
The mail server could not deliver mail to firstname.lastname@example.org. The account or domain may not exist, they may be blacklisted, or missing the proper dns entries.
------ This is a copy of the message, including all the headers. ------
Test what's wrong, disable all your other plugins and switch to the default WordPress theme such as Twenty Twelve or Twenty Eleven for a bit. If the problem persists with the default theme and all other plugins disabled, please contact us.