In order to investigate the issue, please provide the scheduling log from your execution server. If it contains a lot of entries you can clear it and trigger the issue again. Then export the log to Excel or Text file and attach it to this ticket(in a zip file). This article
explains where to find the log.
If you're using your own SMTP server, please keep in mind that you need an email account that has the necessary rights to communicate directly with the SMTP server. Just any regular account will not suffice. Please recheck this with your system administrator.
You can also try to use Gmail SMTP server for email results to see how this will work for you. Here is an useful article
with the settings.
All the best,
the Telerik team