multiple MX records

[admin at homemachine.net (Tom)]

Hi all,

I have 2 servers, 1 is DNS master and the other slave for a domain, if I 
was to add a second MX record using the master servers IP for the domain 
thats hosted on the slave would mail be held on the masters mail queue 
if the slave was down?
I am wanting to have mail held when the slave is down so it gets 
delivered immediately it comes back up instead of being held on other 
servers for up to 8 hours before I get it.

hope that makes sense.

tom