Our client started to use the Server Side Sync via CRM and have some workflows that send emails.
The field From in the email is a system user (Admin) that sends emails to users. We thought that adding the the option to “send as” would work but, actually didn’t.
So we decided to add the option in the Security Rolls that allows to send email as another user (Business Management -> Miscellaneous Privileges)
This allowed the email to be created and sended from CRM.
Hope this helps!