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 server is because the Mail Backup for your domain name is at Monitoring Mode. With such settings, we monitor your email server constantly and will greylist emails come to our server while your mail server is available.

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 email should never deliver email to secondary MX while primary is available, even so, the sending server should instead retry to deliver the email to primary after getting our greylisting rejection. 

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 due to network path failure from the sending server to your server (while our server can reach your server); however, such chances are rare. To totally avoice bounce back from our server, you can turn off Monitoring Mode but the trade off is that will greatly increase spams forwarded to your server. We suggest that you keep Monitoring Mode to fence off Spam.