Thanks for your reply.
Here's what I have determined, I'll let you form your own hypothesis:
When we configure the global email notification settings (in OnTime 2008 Windows) so that the email server does not require authentication most people are able to send email. This one user however can't send email when the OnTime 2008 is configured that way.
When we check the "server requires authentication" checkbox and enter one of our login identities and passwords in the global settings for OnTime Email Notification, it seems to work for him.
Why would others be allowed to send email without authentication and only the one user require it? Strange.