If that error message is to be taken literally, the receiving server's DNS is broken.
It's not broken at all, and that is only one possible explanation in a long list of possible causes for DNS queries to timeout.
On my MX, his domain resolved when he posted the error, so the 450 error msg was temporary.
This is a mild but non-negligible liability when relying on SAV
SAV is not the reason for his rejection. reject_unknown_sender_domain is a separate test from reject_unverified_sender
--I'm not saying it isn't fine to reject on SAV, just that all of your local DNS ducks need to be in a row for it not to suddenly bite you.
If DNS and/or us-to-them connectivity doesn't work, then ALL bets are off, not just SMTP.
Len
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