UNABLE TO SEND: One user gets systematically bounced, mail transaction fails, misleading error.

UNABLE TO SEND: One user gets systematically bounced, mail transaction fails, misleading error.

One of our users gets systematically bounced back. Contrary to what the error received states, the user does indeed exist.

Examples follow. The user tried sending just a few emails today.
Is there anything we could do on our side? This seems to be an issue with Zoho mail, in my opinion.
Delivery to same target email from another user on our very same domain work as expected, the issue seems to be linked to this particular user of ours.

Best regards and thanks,
   Michele Fadda

A) 2nd bounce back
== == == == == == Posta inviata == == == == == == 
Da :  mailer-daemon@mail.zoho.com
A : < stefania.cara@fwlab.com>
Data : ven, 03 ott 2014 17:50:03 +0200
Oggetto : Undelivered Mail Returned to Sender
== == == == == == Posta inviata == == == == == == 

This message was created automatically by mail delivery software. 
A message that you sent could not be delivered to one or more of its recipients. This is a permanent error.  

mpiacara@alice.it Error, ERROR_CODE :554, ERROR_CODE :Transaction failed 


Received:from mail.zoho.com by mx.zohomail.com 
    with SMTP id 14123513572629.580577848933103; Fri, 3 Oct 2014 08:49:17 -0700 (PDT) 
Message-ID:<148d6aa7507.10a1 2077c17404.5932912978401974130@fwlab.com
Date:Fri, 03 Oct 2014 17:49:17 +0200 
From:"stefania.cara" < stefania.cara@fwlab.com
User-Agent:Zoho Mail 
To:" mpiacara@alice.it" < mpiacara@alice.it
Subject:Varie 
Content-Type:multipart/alternative;  
    boundary="----=_Part_24674_1015178727.1412351357197"

B) 1st bounce back

Oggetto : Undelivered Mail Returned to Sender
============ Posta inviata ============

This message was created automatically by mail delivery software. 
A message that you sent could not be delivered to one or more of its recipients. This is a permanent error.  

segreteria@asipse.it Invalid Address, ERROR_CODE :550, ERROR_CODE :Verification failed for < stefania.cara@fwlab.com
            No Such User Here" 
            Sender verify failed