Setup: Primary domain is blogs.foobar.com, used for installation etc. All blogs run under their own respective domains, e.g. Blog A has blog.example.com.
Posts in Blog A show as trackback url http://blogs.foobar.com/htsrv/trackback.php/123, not http://blog.example.com/htsrv/trackback.php/123.
Attempts to send a trackback to that url fail with the error message
Response: No valid trackback response. Maybe the given url is not a Trackback url. ""
If I use http://blog.example.com/htsrv/trackback.php/123 as trackback url instead, it all works.
Seems like the per-blog domains should be used, not the general installation url.