The most common issues are caused by an incorrect DNS server address entered by the user. If you received a "Failed to query MX server of domain.com" message, check if the email address format is correct. A right format looks like: aaa@domain.com. If it is correct, go to Settings > Detect DNS in order to detect your local DNS settings and check the troubleshooting provided by FlashEmailCast. If the problems persists, you need to contact the developer.