Domain does not exist 450.4.1.8 when sending email from exchange 2010 mailbox

I have an email server with 4 domains on it. Three out of the four are working but the fourth is getting the error stated above.
All the domains are registered but different registrars.
I turned on protocol logging but it only verified what I already knew - domain does not exist.
From the send connector, mail goes to a smarthost that is also functioning properly.
All assistance appreciated
FlyboyExch60Asked:
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Darrell PorterEnterprise Business Process ArchitectCommented:
Does the domain in question have both an internal and an external MX record?
Does the domain in question have both an internal and an external SPF record?
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FlyboyExch60Author Commented:
Misconfigured MX record on registrar's site - they all seem to do it differently
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