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Correct, it's not exactly the same thing.
A backup mail server is just another mail server with it's own MX record in your DNS. YOur primary mail server has the lowest number, backup a higher one like this:
mydomain.com MX 14400 10 mx.mydomain.com
mydomain.com MX 14400 20 mx2.mydomain.com
You can have as many mail servers as you wish, this is exactly how services like gmail have dozens of mail servers if not more.
If a mail server cannot connect to the first MX record it will try the second, then the third and so on.
If you list your mail servers with the same "number" they will "round robin" load balance with the mail server handling a request being selected at random.
ex:
mydomain.com MX 14400 0 mx.mydomain.com
mydomain.com MX 14400 0 mx2.mydomain.com
You "could" set up more than 1 spooling server to give a fail over solution that also has some client side redundancy.
Still, unless the clients are configured to connect remotely or you use a cluster for webmail even though your mail is on the server if webmail is down and there is no remote pop/imap access the users won't see that email until it comes back up.
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-= Software / Systems Architect and Server Geek =-
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