What happens when both nameservers are down?

[Venkatesh MaduraiSubramanian]

I have a question and could not find a suitable answer when i searched
for it a few times.                                                   
                                                                      
A friend is running his own 2 nameservers, on different IP's but same 
machine. The two IP's have different FQDN's. Bind version is 8.2.3 on 
Red Hat Linux                                                         
                                                                      
Normally, when the server is down, the DNS servers around the globe   
lose the IP and other info for the domains hosted by the name server. 
                                                                      
I don't know if adding a $TTL line at the top, before the SOA records 
would result in the DNS servers of the world caching the info for the 
value                                                                 
specified in the TTL. Are there other ways to do this?                
                                                                      
The main objective is to ensure that emails do get delivered (most    
SMTP would try for 5 days). If the server is down and IP/MX info are  
missing then the emails will definitely bounce.                       
                                                                      
Idea: Would specifying my hosting provider's secondary name server    
help? (this is normally running more frequently than my server)       
                                                                      
Thanks in advance                                                     
                                                                      
Venkatesh