Thanks again for the response!
I think you’ve confirmed my suspicion that this is an issue specific to hosted Discourse sites like ours. I don’t know what level of effort would be required to have such sites verify that the …discoursemail.com addresses entered in this field are valid, but such a feature would’ve saved me a fair amount of time and frustration over the past several years of setting up new mailing lists and aliases, wondering why they don’t work. I imagine it would help others as well.
Alternatively, even a little tool tip on that field for a hosted site indicating that a legal address must be slug+...@discoursemail.com or ...@slug.discoursemail.com would go a long way. Though I don’t know whether making that tip specific to hosted Discourse sites makes that approach unworkable.
-Brad