Quote Originally Posted by JohnnyT View Post
I just wonder if there is a group of files that control this email behavior that I could compare to a clean version of 1.3.9h? I don't know enough to determine which files would be the important ones to checkout.
JohnnyT,
The primary files involved with sending order-status-change emails from the admin are:
/admin/orders.php
/includes/functions/functions_email.php

However, unless you've changed those files, I suspect your problem is more with bad email addresses than with the code.
Does this happen to EVERY customer? Or just selected ones? How many have you tested it with? What were the email addresses? ie: any particular pattern with those email addresses? all free email services like gmail/hotmail/yahoo? Or actual real email accounts on normal domains? Is it related to certain order-status settings specifically? Are these customers' emails on the email-newsletter list? Have they selected TEXT as their email preference, or HTML, or something else?
etc etc etc

Given that the behavior you are reporting is abnormal and not how the system is coded or designed, it's crucial that you provide as much information related to the bigger picture as possible, if you hope to get meaningful help.