Does anyone know if there is a way to retrieve a list of all "From Email Addresses" used in Email Campaigns on our Marketo instance?
"Champions" is for questions about the champions program. You's belong to "products" and you should move it there to get max exposure and increase the chances that someones answers.
The short answer to your question is unfortunately that there is no possibility to get such a list.
Vote here: Missing info in email performance report