You cannot send email 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. Error in CRM 2013/2015

Was getting the below error in one of the custom workflow activity we were using for sending email.

To fix the issue,

Login to CRM with the user selected as From.

Open Set Personal Options box

And check the check box “Allow other Microsoft Dynamics CRM users to send email on your behalf”

Hope it helps..