I recently was working on ad-hoc comms to get people to register for an upcoming event. I created a Contact List of Registrants to later send specific details leading up to the event but was still sending comms to get people to register. I was not able to select that Contact list to suppress against my Mailing List -- I only could select a suppression list. I needed to create a new suppression list and I don't think that is necessary. Please allow Contact Lists (or even queries) to be selected as well to suppress against your mailing lists.