Do you have a need for two mail servers? SMTP is a store and forward protocol, and is normally quite resilient to outage of up to one or two days.
I am not sure why you have four MX servers in your diagram. You only need one mail domain per MX. Multiple domains can point to the same MX. For example
mail2.example.com can support any number of domains. (NOTE:
mail2 sub-domains are service domains, and are usually different than the hostname the service runs on.) The additional domains need to be either virtual domains, or domains with independent MDAs (Mail Delivery Agents). This may be the purpose of the servers you listed as
server 1 and
server 2, or are these the servers you intent to use as MXs.
If your traffic is balanced more to the outgoing direction, the it may make sense to have an outgoing MTA (Mail Transfer Agent) and a separate MX server for incoming messages.
- one main mail-server + a backup-MX that only queues mail if main is down
PRO: I assume that you intend to use the main mail-server as your MDA. Having only one MDA makes life simpler.
CON: The backup-MX is likely to be used as target by spammers. If it can't fully validate incoming messages when the main mail-server is down, you may need to silently drop messages arriving there or generate back-scatter email.
- two 'dedicated' (possibly identically configured) mail-servers that work as end-points for all domains.
PRO: Having two servers increases your redundancy, and may allow you to balance your load. If the two servers are acting as MDAs, then you need either shared storage, or a method to balance users data across the two servers.
CON: You should have different configurations for the two servers, at a miniumum different mail domains. If you have separate servers acting as MDAs, the configuration differences can be relatively minor.
Running two active MX servers adds additional issues in addition to the issues of running a mail server. Until you have mastered running a single mail server, it may be better to stick with one. Once you have mastered running one server, you should be able to decide which way to split the load for your organization.