Mail Question

[Chong Yu Meng]

Ok, here's my first question on the list :

Scenario : Company XYZ, which I used to work for, deployed a server in a 
certain local ISP. I wrote a Java servlet that tries to mail the results 
  of  a form to the server administrator, along with an acknowledgement 
email to the person who filled out the form.

Problem : The mail will be delivered if the recipient has a email 
account on XYZ.com (i.e. fbloggs at XYZ.com), but if the recipient belongs 
to some other domain, the mail will not get delivered.

I'm thinking that this is probably a mail server problem, and, in fact, 
I have found that other companies that hosted with this ISP have similar 
problems. The mail server does not appear to accept relaying.

Since I have already left the company, getting access to the error logs 
is not really an option.

After discussing this problem with the ISP, I was told that one 
workaround is to install an SMTP agent on my server.

My question is this :
Was the engineer just blowing smoke at me, or does that actually work ?
If the MX record of XYZ.com points to another server (and I can't change 
the record), and I install sendmail or postfix on my server, and point 
my servlet to "localhost" as the SMTP server, will mail delivery 
succeed, even if mail relaying is denied ?

Any help clarifying this would be greatly appreciated !

Regards,
pascal chong