From today, some clients could not send the emails to us. The error message (NDR) shows below, Delivery to the following recipient failed permanently: email@example.com Technical details of permanent failure: Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 550 550 Unrouteable address (state 14). But I check there is no error report at my server. Does anyone here help me to fix this issue? Thanks a lot.