CRM System Setting: Allow messages with unresolved email recipients to be sent
Verify below two settings:
Go to CRM –> Settings –> Administration –> System Settings
Click on “E-mail” tab and scroll down to “Allow messages with unresolved email recipients to be sent”
Approve the primary e-mail address of the user through which id you want to send the e-mail.
Note: In case of CRM on-line after approving the e-mail address it takes few hours for CRM to allow sending emails.