The built-in subscriptions are only available to users. However, you can always use a little scripting in conjunction with the command line utility, TABCMD, to email pdf or image versions of dashboards to any number of email addresses. This article gives some detail: http://kb.tableausoftware.com/articles/knowledgebase/using-tabcmd
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Or just set up an email distribution list on your mailserver, and set up that email address with a Tableau Server license.