That may work as well. The reason that I say to just unset $_POST[‘acf’] is that this is what acf_form_head() does.
1) Verify the nonce
2) Validate the values
3) Runs your pre_save_post filter if any. I’m assuming that you’d put your code to send an email into this filter
4) Calls acf_save_post()
The first thing that acf_save_post() does it to see if $_POST[‘acf’] contains anything, if it’s empty then it returns without saving anything.
The only place this will be a problem is if there is an error during the submission process, for example they didn’t enter an email address into an email field.
Like I said, your solution might work as well, you just need to remove the line that calls acf_save_post()
