smtp relay for external domain

[djingo]

Dear all,
we are having a problem sending emails from our zimbra server to an external domain. (mail keep stuck deferred queue)
Tracking down the problem we discovered a routing problem of the ISP.
To get a temporary fast resolution we would like to set a relay smtp for all mails sent from our zimbra server to this destination domain. Is this possible?
I have searched the forum but found only the configuration to set an external relay for all outgoing mails and not just for a single destination domain.

Thank you for the help and time

Marco