Backup MX Server

Question

I have my domain with an ISP and all is well.
I was wanting to set my domains DNS records so that I get copies of all emails, sent and recieved on this domain, to my mac OSX 10.7 Server, as a backup.
I have OSX Server 10.7.latest setup and ready to go with the Domain name, IP, Reverse DNS, etc and email, user account, settings.
Is it simply a matter of creating another MX record in my Domain Names MX record, at the ISP, to point to my OSX 10.7 Servers external WAN IP number?
The current MX mail record has a priority of 10
I have a few other OSX 10.7 Servers running as primary domain mail servers, so I am familar with a working setup already.
Thank you,

Answer 1

Yes, this is possible.  But you will not have a unified message store and you will need to do a redirection of your client systems.  Here is the reasons.
First, if you have mail server 1 at mail1.domian.com and a second at mail2.domain.com, it is easy to create the same accounts on these systems.  Thus, MX failover will "work."  But unless they are accessing the same message store (some SAN storage), you will not have a unified message store.  Thus, is mail1 is running smooth and has 10s of GB of mail and goes down, then mail2 will start receiving messages but the historical content will not be present.  The results on the client (IMAP, Exchange, etc) will be unpredictable and merging the content once mail1 comes back on line will be painful to say the least.  Again, this is solved via a clusters mail solution accessing common storage.  Apple used to support this with a click.  Ah, the good old days. 
The issue with a clustered server though is that both head nodes tend to be in the same place since they need access to the same storage.  And then you are back to a single point of failure... the storage. 
Now, the second point is that you will have no means of handling the client during the failure of mail1.  Recall, MX records are for mail delivery routing, not from IMAP/POP/Exchange client connections.  If this is the case and you configure your mail clients to connect to mail1.domain.com and this server goes down, they will not magically connect to mail2 unless you modify all client configurations.  But again, we are back to the message store.
If you are looking for backup, look at services like MX Save.  They provide an SMTP backup service that will collect all undeliverable mail while your primary server is down and then deliver it when you are back up.  Plus, the mimic your server allowing web based or client based access during primary server outage allowing your users to continue to function as if the primary server were still running.
Building your own mail cluster is difficult enough without the limitations of OS X.  If you are really looking to do this you ought to look at another platform.  Even Kerio is really bad at this.  Especially when you are considering multiple locations.  Keeping the storage in sync is almost impossible on a high traffic server.