Sender Verify Callouts - Question

[nathacof]

When sender verify callout is enabled in WHM, is Exim supposed to lookup the MX record for a domain and issue the callout to the server listed as the primary MX record?

The reason I ask is because westnic.net's email server is connecting to the relay host (a cluster of barracuda firewalls that don't accept email for any domain), and one of my users is getting bounce backs when trying to send to any domain hosted on their server. Westnic.net's support has confirmed that their server is trying to verify the email address by connecting to the barracudas, rather than the primary MX record for the domain.

The only thing I can think of is that there may have been a version of cPanel which handled these callouts incorrectly. If not why else would exim be trying to connect to the sending server unless something is wrong with their installation?

Any information is greatly appreciated.