As per this change, numeric links are considered "invalid" and prevent posting with the message:
Your post contains a link to the invalid host '184.108.40.206'.
Please correct it by specifying a non numeric domain or wrapping it in a code block.
This is a somewhat non-standard meaning for "invalid host", and annoys me a bit because something like http://nonexistenthost.example.com/ (an NXDOMAIN), what I'd consider an invalid host, isn't prevented when posting. I'm not the only one who's ever been surprised by this, especially as noted there, this can happen for text that was automatically link-ified, not a link that I actually made.
Maybe the message could be changed to be more meaningful, but I think that the biggest help would simply be to link to the meta page announcing (and defending) the policy. Apart from giving anyone who's riled a fuller explanation, this would also facilitate continuous feedback. As it is, meta can be difficult to search for policy announcements if you don't know exactly what you're looking for. Providing the relevant links makes the information hugely more discoverable.