I know for certain that Postfix and Smail will immediately bounce a message when the domain is authoritatively non-existant. I'd be very surprised and dismayed if sendmail and all other true SMTP mailers did not do exactly the same thing.
Sendmail most definitely does not, instead treating the error as a transient error, issuing an SMTP error code in the 400 series, and continuing to try to send the mail for up to five days (the default), or whatever the mail server admin configured for that particular server.
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. Steinar Haug, Nethelp consulting, sthaug@nethelp.no