After filling out of Common Form the Acknowledgement email either does not send or is sent to the wrong email address.
If the user who submits the form is a logged-in user in NetCommunity, then the acknowledgement will be sent to that user's primary address in NetCommunity.
If the person filling out the form is not logged in, the acknowledgement will be sent to the email field on the form itself. To prevent the wrong user from receiving the acknowledgement, make sure to have only one field on the form whose data type is 'Email'.
Note that if you'd like to collect other email addresses, you can do this by including those fields as data type 'Text'. You can also apply the validation of type 'Email' to those text fields to ensure that users enter email addresses in the correct format.