PHP Email Script - Server Not Notifed

I've been getting "Server not Notified" emails from my shopping cart company eJunkie every time I make a sale on my website. The php script in the error notice generates an email purchase receipt that's sent to my customers that I'm copied on. Here's a snippet from the email: This is a notification that for the transaction 27C4272447824750A, we were unable to notify your server at [login to view URL] as we we received the error: HTTP Error 500 Rackspace is my web hosting company. I called them to ask what may be causing this. They helped me investigate it. What we think is the cause of the problem is that the php script was originally written when I used Rackspace for my business emails. About 6 months ago I moved my Rackspace email to Google Apps for Work. The Rackspace team thinks the problem is the php script is calling on my old Rackspace email address to generate the sent from email address. They think it needs to be changed to the Google Apps for Work email server. I'm guessing that this is the problem so any help you can provide to diagnosing the problem and then fixing it would be great.