is that I get w/ telnet and mail shield enabled (including NNTP). Note that a non-public news server which apparently also checks for IP address is something not useful for reproduction and debugging purposes.
OK, I think it comes down to using telnet over port 119 as the Mail Shield will be redirecting 119 and it will be expecting NNTP protocol and that may be what the problem is.
Rather than stop the Mail Shield you could edit the settings and disable the NNTP scan, if you aren’t going to use NNTP protocol but telnet that is about all I can suggest, see image of Expert Settings, Main Settings screen.