bad emails not bouncing

Had an interesting problem recently which I wanted to see if anybody else has ever experienced; I hope I can explain the situation clearly....

I have a script I wrote in PHP used for sending an email to a mailing list which connects to a MySQL database and pulls a list of email addresses which it then sends an email to one by one in a loop.

The script resides on a Linux server and is connecting to a MySQL database on another server (which is a windows server). The "from" email address identified in the message the script sends has an account on the remote windows server but not on the Linux server I used to send the email.

It seems that because the server being used to send the email messages is not the same as the server which actually has the email accounts on it causes any bad email addresses in the mailing list to not trigger bounce backs. Could this be possible? Is there a way I can force bounced emails to be sent to the reply-to address of the message? Would sending the email messages using SMTP help?

For what it's worth, the way I discovered that this might be the problem is by sending an email to a non-existent account using Outlook. If I sent a message to the fake address using my ISP's mail server, I would not get a bounce back. However, if I sent the message to the fake account using the mail server which my email account is hosted on, I would get a bounce back.

Hope all of that makes sense!