Customers get the error below when purchasing.

Email Error: SMTP Error: The following recipients failed

I can send emails to these customers with the admin "Send Email" page, but they get this message when ordering. That makes me think that the order process uses a different email path than admin. Any ideas what's wrong? (BTW, I have visted the troubleshooting page, with no results.)

Deployment info:
Zen Cart: 1.3.8a
Windows Server 2003
MailEnable SMTP program
SMTP Authentication enabled and working
PHP 5.2.9-2

--ray