smtp reports domain not registered

[dulatoag]

Hey:

I have a client using the Plesk server as her outbound smtp server. Occasionally when she e-mails another recipient in the same domain, she gets the following error returned:

MailEnable: Message Delivery Failure.

The following recipient(s) could not be reached:

[SMTP:[email protected],]: The message could not be delivered because the domain name (healthalliancepo.com,) does not appear to be registered.

No one else using a different outbound smtp server not affiliated with the plesk server gets this error. The plesk server hosts their pop3 mail accounts.

I am not using plesk's dns server for resolution from the server itself - all domains and mx records are hosted with a third-party provider. Any help truly appreciated.

thank you.
Darren La Padula