Yes, we are in exactly the same situation. Our mailserver receives a lot of mail for various domains, and forwards to various users with xtra addresses. We are getting the exact same error – deferred etc
What I'm seeing is rather interesting, all my mail forwarded to xtra get
accepted by mx1.tnz.mail.yahoo.com.
Around 75% of the mail forwarded to mx2.tnz.mail.yahoo.com gets the
"Deferred: 451 VS2-MF Excessive unknown recipients"
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Jean-Francois Pirus