CRM 4 : Workflow doesn't send emails

Hello all,

I've followed the steps mentioned in this post : http://social.microsoft.com/Forums/en-US/3de4b605-1a7a-4592-a200-7bab5b6e1d2f/crm-40-workflow-on-custom-entity-to-send-email-on-due-date-column

To add a DueDate field to the Leads and send an email to the owner in the DueDate

But when the due date occurs nothing happens.

I have :

1st STEP : Worklflow -> Timeout -> Equal -> Lead:Due Date

In the Form Assistant , I have 0months, 0days, 0hours, 0minutes After Lead Due Date

"then"

       -> Send Email

I created a new lead with the current Date in the Due Date

How may I check if that's a problem with the Workflow settings or with the email configuration?

No errors in the event viewer.

Is there another place to check for errors?