Koken sends emails when recovering lost passwords as well as messages submitted through contact forms on your site. For this it uses your host server's PHP installation. For most users this should work okay, but email on self-hosted servers is prone to errors, so depending on who your hosting provider is you may experience trouble receiving email.
If you do, we recommend installing our free Email Endpoints plugin. This plugin will route all emails sent by Koken through your own SMTP server (including Gmail) or Mandrill for more reliable results. Either of these options will likely resolve any email issues you're experiencing.