smtp bounce, how to fix?

Hi to all,
I am running a small mailing list with xmail and for one recipient I get a 
bounce e-mail.

My host is mail.myserver.it and the sender is [email protected]. The 
mailing list address is [email protected].
I guess that the remote SMTP complains about the fact that 
myserver.it<>senderdomain.com.
It's the only SMTP I've met so far that complains about this problem.

There's something I can to to let the recipient receive the mailing list?

Thank you in advice for your help.

Here follows the bounce email.

-----

[<00>] XMail bounce: Rcpt=[[email protected]];Error=[550 5.7.1 
<[email protected]>... Sending host domain must match message 
originator!]


[<01>] Error sending message [1292234962517.1d5c.21f8.2400d7.mail] from 
[mail.myserver.it].

ID: <S166A>
Mail From: <[email protected]>
Rcpt To: <[email protected]>
Server: <smtp.externaldomain.com> [x.x.x.x]


[<02>] The reason of the delivery failure was:

550 5.7.1 <[email protected]>... Sending host domain must match 
message originator!

[<04>] Here is listed the message log file:

[PeekTime] 1292245563 : Mon, 13 Dec 2010 14:06:03 +0100
<<
ErrCode   = -77
ErrString = [RCPT TO:] not permitted by remote SMTP server
ErrInfo   = 550 5.7.1 <[email protected]>... Sending host domain 
must match message originator!
SMAIL SMTP-Send MX = "smtp.externaldomain.com." SMTP = "mail.myserver.it" 
 From = "[email protected]" To = "[email protected]" Failed !
SMTP-Error = "550 5.7.1 <[email protected]>... Sending host 
domain must match message originator!"
SMTP-Server = "smtp.externaldomain.com."
 >> >>

[<05>] Here is listed the initial part of the message:

Reply-To: [email protected]
Received: from someserver.somewhere.com ([k.k.k.k]:4605)
     by mail.myserver.it with [XMail 1.27 ESMTP Server]
     id <S166A> for <[email protected]> from <[email protected]>;
     Mon, 13 Dec 2010 14:06:03 +0100
Received: from SNT137-W28 ([y.y.y.y]) by someotherserver.somewhere.com with 
Microsoft SMTPSVC(6.0.3790.4675);
      Mon, 13 Dec 2010 05:05:56 -0800
Message-ID: <[email protected]>
Return-Path: [email protected]
Content-Type: multipart/alternative;
     boundary="_43928274-5404-452a-9bd5-3e507dc6b731_"
X-Originating-IP: [z.z.z.z]
From: john doe <[email protected]>
To: <[email protected]>
Subject: this is the subject
Date: Mon, 13 Dec 2010 14:05:57 +0100
Importance: Normal
In-Reply-To: <[email protected]>
References: <[email protected]>
MIME-Version: 1.0
X-OriginalArrivalTime: 13 Dec 2010 13:05:56.0924 (UTC) 
FILETIME=[7B130FC0:01CB9AC6]



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