Both company's accounts (@amismet.com) were working perfectly yesterday. Today, I recieve two kinds of errors:

While sending, I recieve: 

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. 

tecnicanorte@ciudad.com.ar  Invalid Address, ERROR_CODE :550, ERROR_CODE :5.1.1 < tecnicanorte@ciudad.com.ar >: Recipient address rejected: ciudad.com.ar 


Received:from AndresPC (8-30-235-201.fibertel.com.ar [201.235.30.8]) by mx.zohomail.com 
    with SMTPS id 1424871350629460.97880094335983; Wed, 25 Feb 2015 05:35:50 -0800 (PST) 
Message-ID:<006201d050ff$f7559cd0$e600d670$@amismet.com> 
Date:Wed, 25 Feb 2015 10:35:45 -0300 
From:AMISMET S.A. - Andrés N. Fernández < afernandez@amismet.com > 
To:< tecnicanorte@ciudad.com.ar > 
Subject:RE: PAGO FACTURA 
Content-Type:multipart/alternative; 
    boundary="----=_NextPart_000_0063_01D050E6.D2099D50"

While someone try to send me, they recieve:

This is an automatically generated Delivery Status Notification.
Delivery to the following recipients failed.
administracion@amismet.com
--Archivo adjunto de mensaje reenviado--

From: andresnf86@hotmail.com
To: administracion@amismet.com
Subject: RE: asd
Date: Wed, 25 Feb 2015 22:12:21 +0000

I can't find the problem!

I would appreciate help! Thanks!

Thanks!!!