Cannot Resolve E-Mail Relay Error When Sending to Certain Domains

We host our own e-mail and use Exchange Server 2003 in addition to TZO's Outbound Mail Relay Service.  There are a small number of domains we cannot email to that get bounced with the following error:
    SMTP error from remote mail server after RCPT TO:<>:
    host []: 550 5.7.1 Unable to relay for

I can't tell if this is an issue with my MX records for our internal domain or if it is an issue with TZO's setup.  It happens to us probably 1% of the time but it's still irritating and I'd like to fully resolve it.  Thanks for any input or direction.
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the issue is not from your side, the one that is rejecting these emails is [] which is, I assume, you TZO

definitely not from your sever

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rohnlawadminAuthor Commented:
Actually that's the recipient's domain.  Thanks for confirming though.  At least I can tell the powers that be, that it's not my end.
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