DKIM signing for your domains lets email servers trust the fact that emails in your domain are coming directly from you, and that they haven't been altered en route. AppRiver can help you enable DKIM signing for your domains.
StreamSend does support DKIM signing for your domains as a part of their annual package. In order to enable it, you can contact their support team.
To learn more about DKIM and how to manually configure it, view our what is DKIM guide.