Very rare occations, email to your domain may get

ERROR:4.3.0:451 Please deliver zedd.org emails directly to primary MX that is up at this moment.

The reason you get such bounce back from our service is because the Mail Backup for your domain name is at Monitoring Mode. In such mode, we monitor your email server and will greylist any emails come to our server while your mail server is working.

The purpuse of Monitoring Mode is to fence off Spam since secondary MX is usually the "honeypots" for spam. As you might not know that almost all spamming servers target at secondary MX even thought the primary MX is available. The reason secondary MX is targeted as spam "honeyposts" is becuase they usually has little or no security checks compared to the primary MX. Under Monitoring Mode, our mail server will return greylisting code 4.3.0 to the sending server while your server is available. A good sending server should retry to delivery emails to your server instead. 

The Mail backup at Monitoring Mode will reduce at least 75% of spams hitting your mail server. At very rare occations, a good email may get bounce back from our server probably duo to network path failure from the sending server to your server (while our server can reach your server); however, such change is rare. To totally avoice bounce back from our server, you can turn off Monitoring Mode but the trade off is that will greatly increase spam forwarded to your server. We suggest that you keep Monitoring Mode to fence off Spam.