If contacts are opted-in to an email subscription with their secondary email address, there is currently no way of filtering for these in lists - even though that specific contact is in fact opted in.
Ideal functionality: list filter could pick up on secondary email subscription, and ensure that when I come to send an email using this list, it is sent to the correct address.
Opt-in is viewed on an email address by email address basis -- not by ‘contact’. Non-primary email addresses aren’t considered when sending, nor are they considered is lists.
- see Add multiple email addresses to a contact
Thanks Frank. I understand why this is happening - my suggestion is precisely that non-primary email addresses should be considered when sending / be filterable in lists.