While the AL lists are well-maintained and mostly spam free, sometimes a few get through, as on the arch-dev-public list today. I don't know if list admins can specifically mark a message as a spam from the server side but there should be a way to do so. I could mark as spam from Gmail but then there's a chance other posts in arch-dev-public would also be classified as spam (that happened before with me with aur-general).

This spam could be because of some server problem, a few months ago this kind of spam used to come (from arch-dev-public@archlinux.org) and Aaron had fixed it.