[lug] OT: Dealing with Unreliable Mail/DNS Servers

[Jeffrey Siegal]

You ought to be able to set things up so that a domain that simply 
fails to resolve at all, as opposed to a domain for which you get an 
authoritative "no such domain" answer, is not bounced but refused with 
a retryable failure code (4xx).  This will cause the sender to try 
again later, at which time you'll hopefully be able to resolve the 
domain.