Marcin Zajączkowski created an issue #468:
Trying to write a message the gnome-shell-list mailing list, I've got:
The email address you supplied is banned from this mailing list. If you think this restriction is erroneous, please contact the list owners at gnome-shell-list-owner gnome org.
I think that restriction is erroneous as @wp.pl is provided by Wirtualna Polska one of the oldest "internet companies" in Polska. It hosts several millions of email accounts and is one of the most popular email providers in Poland (the link in Polish about their webmail "views" compared to GMail - I couldn't find an old article about precise number of hosted mailboxes).
In the context of the ongoing migration to Discourse it's not a top priority, but anyway, some old fashioned people from Poland could still want to post to the mailing list.