nice cleanup (for which, thanks) but I'm still having a hard time getting a grip on your question. Please clarify "The program was executed in the machine B and it is not able to send the email.".

Do you mean that you are trying to execute the same script on both Machine A and Machine B to send to a third machine (where USU1 exists)? or that "B" fails to execute Mail::Sender to reply to A or ???

or (perhaps an issue) are you sure the w2k Pro machine (B) has smtp server. I suspect that's stock in the w2k Server machine, but suspect it is not installed on w2k Pro. M$ didn't distinguish the two solely to bump up its product count - ie, is there an SMTP server on B?

If you're trying to have "B" talk to "A" when you get the error, do you have an IP issue? Documentation says

smtp
     => the IP or domain address of your SMTP (mail) server

    This is the name of your LOCAL mail server, do NOT try to contact 
+directly the adressee's mailserver! That would be slow and buggy, you
+r script should only pass the messages to the nearest mail server and
+ leave the rest to it. ...
[download]

I'm not even sure I'm asking you for the right info, but since I see no other attempts to respond, I suspect others are having difficulty parsing your problem, too.

UPDATE: bang-one, in CB, suggests: "...have him confirm that he can connect to the other server. A simple "telnet ip 25" should tell him whether or not it's a factor outside of the code.
<!1> Which I suspect it is, considering the script works on another machine.