The SharePoint site in question is internet facing extranet site built on SharePoint 2103 Foundation. There is no Active Directory or Exchange Server and the site members have email addresses from different domains (Google, Yahoo etc.). Now I would like to have some kind of "dynamic" distribution list based on group membership within SharePoint. So that when I email firstname.lastname@example.org, the email is relayed to all members of my site. How can I do that?
Currently the email is hosted on my Linux hosting account which has Horde, Roundcube and Squirrelmail. I can also install phplist if required. I am open to any suggestions that may require an additional component. However, I have unlimited-user SharePoint hosting and so any email hosting (like Exchange) where we have to pay per user or per mailbox will not work (unless Exchange can create a distribution list of external addresses and populate it directly from SharePoint, which I am not sure it can).
Ideally, the members of the site should also be able to opt-in and opt-out of newsletters through SharePoint.