We have a partner network of several hundred affiliates, partners etc. When partners sign up, they are given an option to specify the email address preferred for partner communications. Our previous Marketing Automation platform allowed for tokens in the to address within an email template. This allowed us to easily send newsletters to the preferred email (which may be different than primary email).
Marketo seems to only allow sending to the primary email field unless the "Send Alert" flow step is used but that doesn't have reporting around it.