I am Getting max defers and failures per hour while sending Mail

The maximum number of failed mails sent from the domain is set to 50% . If it exceeds that particular limit you will receive the above error.It is the bounce back messages which is notifying the undelivered messages. Our server regularly monitor the emails sent through all email accounts on your domain, and if, over the past hour, more than 25% of the attempted deliveries have failed, outbound email sending will be temporarily be limited and you will receive the above bounce back messages.The block will be released automatically after an hour.

It could be due to following reasons

i)If you send mass mail and haven’t kept your mailing list clean by removing invalid email addresses, you may generate enough bounces to have your mailings limited.
ii)Suppose if your system has been compromised and if it sends spam mails then you will get the above bounce back messages.
iii)If you failed to keep their PHP scripts updated with newest versions, then spam mails might generated from that infected site.

Kindly recheck proceed further.