This may be the full NNTP URL of an independently-run mirror.
For example, the https://public-inbox.org/meta/ inbox is
mirrored by Gmane at
-C<nntp://news.gmane.org/gmane.mail.public-inbox.general>
+C<nntp://news.gmane.io/gmane.mail.public-inbox.general>
Default: none