It's possible that ListMail has become confused with a server's response due to a certain email address being on your list.
To see what's really going on uncomment a line in admin.php to enable SMTP debug messages: 
// debug, shows SMTP messages 
// $bugs = '1'; 
should become: 
// debug, shows SMTP messages 
$bugs = '1'; 
Note that this will output a LOT of SMTP messages when sending email to your list. These messages can only be seen when doing a "Send Email" to your list, not with scheduled email. 
Please let me know the last few SMTP responses when outputting the extra messages and the mailing fails.