I know you said you are using Net::SMTP, but since you tried using sendmail without success I thought I'd give you an example that works for me (with WinXP). Perhaps it will come in handy in the future.

use strict;
use warnings;
use Mail::Sendmail;

my %mail;

# set smtp mail server (IP xxx.xxx.xxx.xxx)
$mail{smtp} = 'mail.server.edu';

# set mail info
$mail{To} = 'recipient1@foo.com';
$mail{CC} = 'recipient2@foo.com';
$mail{bcc} = 'recipient1@bar.edu';
$mail{From} = 'recipient1@baz.org';
$mail{Subject} = 'testing';
$mail{Message} = "hi";

# send the email
sendmail( %mail ) or die $Mail::Sendmail::error;
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HTH