I don't know enough about your template, and how the completed form is used, saved etc. to be able to test this out properly.
But the following may give you something to work with.
I added a check box to a Word document and used the CheckBox Click event to trigger the macro.
If the check box was checked (Value=True), then the rest of the code runs.
It sends an html attachment (it can send other types) but I have found in my limited use of this that html works!
So save a copy of the form as in html file format and use that as the email attachment.
Using html format means that the check box and associated VBA code is not sent.
One problem with this method is that if your outgoing mail server requires authentication, you need a password - which is in plain text.
If you do go down this route, then apart from protecting the code and the form, you may want to set up an email account that is only used for this purpose, so that anyone gaining access to the password cannot access a regular email account.
Private Sub CheckBox1_Click()
If Me.CheckBox1.Value = True Then
Dim strFilename As String
Dim objMail As Object
On Error GoTo ErrHnd:
'copy the current path and filename
strOrigFileName = Me.Path & "\" & Me.Name
'set the temp filename and path
strFileSave = "C:\Temp\EMailTemp.html"
'save a copy as a temporary file
Me.SaveAs FileName:=strFileSave, Fileformat:=wdFormatHTML
're-save as original file, so as to release temp file
Me.SaveAs FileName:=strOrigFileName, Fileformat:=wdFormatDocument
'create the e-mail object
Set objEmail = CreateObject("CDO.Message")
'create e-mail message
.To = "A@b.com"
.From = "C@d.com"
.Subject = "Completed form"
'remove the e-mail object
Set objEmail = Nothing
Checking the check box in my test document did send the document and I was able to retrieve it from my email in box and open the attachment in my browser.