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.

Solution:https://nishantrana.me/2015/07/26/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-201/