Thanks for the reply Zhang. What i mean by backup MX is, its just another IP address with secondary mailbox for all the users that i have created in iRedmail Server. We switch off the server @ night time. So all the mail coming at night will land up in hosting mailbox. In the morning once the server is switched on, i have to retrieve all the mails from secondary mailbox and dump it inside iredmail server's mailbox, so as to enable users view the mails that have come overnight. In case i don't do this, they would have 2 mail boxes and it would create lot of confusion.

Thanks for your Reply Zhang. PFB the answers :
Why shut down the primary server at night time? - we are small company. This practice increases lifetime of server upto 50 % by regularly shutting down.

But we are currently using http://webmail.gojobs.biz as our secendory mx which - itself is a website. We have something like pop3 connection manager that has details like username, password for users with mailbox in their domain. Daily morning, it automatically runs and retrieves all the mails that had landed there, to our mail server and users would get over night mails.  This practice has lots of advantage and also a compliance standard for mail redundancy and security. in-case we use this with iredmail, is there option to fetch the mails from some other provider and re-route it to iredmail inbox?