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.