You can use the Community Broadcast API with the POST method to broadcast mail to a community.
||Value of the href attribute of the <link> element with the rel="http://www.ibm.com/xmlns/prod/sn/community-broadcasts" attribute value for each community.
<?xml version="1.0" encoding="UTF-8"?>
<title type="html">Broadcast mail</title>
<content type="text">Broadcast test </content>
||The UUID of the community you are sending the mail to.
||Created. Indicates that the broadcast was successfully sent. If an error occurs, this header contains one of the error codes.
||Unauthorized. Returned when no authenticated user or no userid, email, or key parameter are provided on the request.