Question : How do I tell if my email is hosted internally or externally?

I have recently started my new job as an Network Admin but unfortunately predecessor did not leave much information on how to setup email. I have a Fedora server that is running Plesk 8.0.0. I have created an account in Plesk and can receive internal mail but I am unable to receive any external and I don't believe it is being blocked by black/white lists as one of my co-workers emailed themselves from the same place but could not send email to my address. I have the error that I am receiving below. I am guessing that my mail is being hosted externally because I do not know of anything else that would hinder just my email address. I also have a firebox x1000 firewall but I am unfamiliar with Watchguards software. Any help you can give would be greatly appreciated. Thank you.

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Hi. This is the qmail-send program at yahoo.com.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.
 
:
64.18.7.10 does not like recipient.
Remote host said: 550 No such user - psmtp
Giving up on 64.18.7.10.
 
--- Below this line is a copy of the message.
 
Return-Path: 
Received: (qmail 23109 invoked by uid 60001); 2 Jan 2008 18:19:14 -0000
DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
  s=s1024; d=yahoo.com;
  h=X-YMail-OSG:Received:X-Mailer:Date:From:Subject:To:MIME-Version:Content-Type:Message-ID;
  b=HtuqSYFet5v15FJtwKzFXNFa8nfNJHtP1t3L3985W70cMHoNRWzWRpgDtDkHHYjt+eGRclE77eNn3JEbgEUu1ApehWb/4OWg1qvEhbmcdyEJHu40ZdMh70vdp2LGpXdjAlyHjCkN/z2jiXfKB+Y0VJ19ST33CeDSNOxqwOXk9Pg=;
X-YMail-OSG: BOGd6sIVM1k1kifBWN5XEbnh6cAWGHPLFby2i18EnofxlyQb5N_kuEFGnhooMwnLodhoVzJ_tYOm9hd.gp5mGzRdpnSsUv9Nt3n_MOcq_Dor7bHN6Oc-
Received: from [69.33.45.50] by web51704.mail.re2.yahoo.com via HTTP; Wed, 02 Jan 2008 10:19:14 PST
X-Mailer: YahooMailRC/818.31 YahooMailWebService/0.7.158.1
Date: Wed, 2 Jan 2008 10:19:14 -0800 (PST)
From: Greg Taylor 
Subject: Re: Test
To: Greg Taylor 
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="0-1939237227-1199297954=:22863"
Message-ID: <[email protected]>
 
--0-1939237227-1199297954=:22863
Content-Type: text/plain; charset=us-ascii
 
Test testing Tested
 
----- Original Message ----
From: Greg Taylor 
To: [email protected]
Sent: Wednesday, January 2, 2008 9:58:53 AM
Subject: Test

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Answer : How do I tell if my email is hosted internally or externally?

The error message points to the fact that user validation is not occurring.

I have looked at the MX records for the domain 'americopy.com' and they point to servers who's primary name server is within the postini.com domain.

Postini is a company that offers mail filtering etc... services. It looks like your new company is using their services. If this is the case, it is likely that they are not aware of this user and are bouncing your mail.

You mentioned that you have recently joined the company and that you created the account, have you let them know about the new user? I am not sure how they update their user validation lists.

Hope this helped,
Colin