Well settings first. Emails are sent TO the users address as set in their WP Profile so first task is to make sure that is correct.
The sent FROM details are (optionally) held in the forum admin > options > Email Settings panel where you can set the components of the from address – so check those are correct as well. if you do not use these options then WP will send them as ‘WordPress@your_domain’.
All emails are sent by the WordPress email routines NOT directly by Simple:Press. The WP forums are filled with email problem reports! As Steve said above – we usually recommend the HGK SMTP plugin which we also use on this site and it most often will solve our users email issues successfully.