In neetoCRM there are couple of places from where you can send email ie
Entity show page ->
Lead bulk actions ->
In send email form you get an option to select smart mail or your synced account mail Id. Both the options would be present if smarts mails feature is enabled and google account is synced. Your preferred type ie smart mail or synced mail would be stored and will be used as default option when you are trying to send email again. You will always have an option to select between smart mails and synced account mail Id.
If only smart mail feature is enabled and google account is not synced then by default smart mail option be selected in send email form and smart mail would be sent to entity ie lead/deal/contact. Similarly if smart mail feature is disabled and google account is synced then by default google account mail id is used to send sync email to entity ie lead/deal/contact.
If both options ie smart mail feature is disabled and google account is not synced then send email option would not be present.