Also, if your email box goes down and is unreachable then the sending server should retry later when it can't get a response. If that box is also your DNS serve

Also, if your email box goes down and is unreachable then the sending server should retry later when it can't get a response. If that box is also your DNS server, and the sending email server is unable to lookup the MX record it will automatically bounce. Having a slave/secondary email server should mitigate this to some extent though. Many applications do plan for unreachable services, but throwing the fault down the stack screws up how the error is handled. You can still use a primary application server box as a DNS server, but I would be certain that it's a secondary/slave server, not the primary server.