MassMailer provides option to make your email template to Read Only so that other users can't edit the template while sending mass emails
Note: This feature will be available only on MassMailer Email Wizard for now.
In order to enable this feature we need to follow the below steps
Click on Setup on the top right corner of your screen.
In Quick Find, search for Custom Settings --> CustomSite_URL-->Manage Button-->Site_URL then we need flag MM Email Wizard Template Read Only field to true/checked---> Save
You can't edit the subject line and email body and there is no Modify Email Templete option and Save As button at all, see below image for your reference 