Issue: Emails are not sending out because other users don't own the workflow.
1. Go to the Process that triggers the email Settings > Processes
2. Open the process
3. Select Process Sessions in the left navigation
4. Select a Process in Waiting Status
5. Open one of them
6. Check if the error message is: You cannot send e-mail as the selected user. The selected user has not allowed this or you do not have sufficient privileges to do so. Contact your system administrator for assistance.
Resolution: The User who owns the workflow need to allow other to send email on their behalf.
1. Go to the Settings Icon and Select Options
2. Click the E-mail tab and check the box Allow other Microsoft Dynamics 365 users to send e-mail on your behalf.