I think I like it better that way. Just because both nameservers are temporarily down doesn't mean the domain doesn't exist. :P
Seems to me you're talking past one another. If all nameservers for a domain are down there *is* no nameserver which can say that the domain is authoritatively non-existent. (OK, you could get a negative caching answer from one of the authoritative servers on the level above, but that's a different issue...)
I agree with Greg Woods - if a domain is authoritatively non-existent, I'd expect a sane mailer to bounce the message.
I'm glad that you don't write code, especially sendmail code.
I'm hoping that some clarification of terms will kill this thread... There is a difference between the following terms: Authoritatively Non-existant domain: A domain for which the servers authoritative for the parent zone report that the domain does not exist. Unreachable Authoritative Servers: A domain which exists and posesses a delegation in it's parent servers, but which authoritative servers are currently not reachable. The former is regarded as a permanent error and mail is immediately bounced by every mailer I know of. The latter is a transient error and is treated as such by sendmail. I would hope it would be treated as such by other mailers. Owen