I’d suggest you to check the *Mail configuration* settings in the *Configuration/Server* configuration page of the Scheduler Web Administration tool. I’d use the *Test connection* option of this page to check if the mail server is correctly configured. In case the test is not successful you can check the logs in *<DENODOHOME>\logs\scheduler\scheduler.log* to get more information about the problem.
In addition, I’d check the connectivity between the Scheduler server and the Smtp server and if you are using the correct credentials.
Hope this helps!