I have my own mail server on a VPS ( 1 GB ) using Cpanel ( Exim MTA )( ns1.example.com ) I want to make it more redundant. In the sense, as email is key service, if it goes down for whatever reason, all your website is at stake. I have one more VPS sever ( 512 GB ) with Cpanel ( Exim MTA ) which is used for Development activities ( NS2.example.com ) All my domain ( example : www.investorstreet.com ) has this two named DNS servers to bring redundancy. I similarly want to bring redundancy in mail service as well. In the sense,If one mail server goes down, other should provide the support. Please can some help me to do so. Both servers are on VPS optimized CPanel.
To provide true email server redundancy you would need each copy of the email server software to share a database where all the account information was stored. A shared database itself becomes a single point of failure unless you install the DB on a redundant SAN which means that apart from your two VPSs you would require two additional dedicated servers set-up as a SAN with some way of keeping the SANs in sync. Real clustering and redundancy costs a lot more money that a couple of VPSs.
To make it more simple, you need to create the similar email accounts on ns2 server and in the DNS zone of the domains on both the server, you need to specify 2 MX records. The 1st MX record pointing to the server where the domains are actually hosted. The 2nd MX record pointing to ns2 server. Make sure the priority of the 1st MX record should be lower than the 2nd MX. BTW, the emails will be scattered in such a situation i.e. if ns1 goes down, emails will goto ns2 but when ns1 comes online, the emails on ns2 need to be synced to ns1 OR to avoid the case of sync'ing, you can configure your email accounts in Outlook. You will have all the emails on your local machine whether came from ns1 OR ns2.