Hi KDillon,
You could disable email form by javascript:
function disableEmailForm(executionContext) {
var formContext = executionContext.getFormContext();
var formType = formContext.ui.getFormType();
// Create new record form type
if (formType == 1) {
var controls = formContext.ui.controls.get();
for (var i in controls) {
var control = controls[i];
if (!control.getDisabled()) {
control.setDisabled(true);
}
}
// Hide editor if you don't want see error.
formContext.ui.controls.get()[5].setVisible(false);
var alertStrings = { confirmButtonLabel: "I know", text: "Please send email by outlook.", title: "Notification" };
var alertOptions = { height: 150, width: 250 };
Xrm.Navigation.openAlertDialog(alertStrings, alertOptions).then(
function success(result) {
console.log("Alert dialog closed");
},
function(error) {
console.log(error.message);
}
);
}
}
It will disable all email form fields to prevent user editing it,

even if user click send button, it will show notification that "The email must have at least one recipient before it can be sent."
The only short is that after the editor be disabled, it will display error in editor content,
you could hide it(I've already added) with formContext.ui.controls.get()[5.setVisible(false); .
Regards,
Clofly