How do I solve my subscriber getting "Sent from an unauthorized email address" error messages
Mail-List.com will only allow members of your list to post a message. They can be subscribers on either your regular list or your Digest.
Sometimes people are on your list with one address, but send from a different email address.
Perhaps they are on you list as email@example.com but are sending from firstname.lastname@example.org
Or they are on your list with their Yahoo address, but posting from their work address.
Ask your subscriber to forward you the error message they are getting
This part of the error message shows what address they are posting from.