chances are what that app does is block the port.
the messages can be seen yourself on an internal network by running this string from the command line
"net send computername messagetext"
or if you have friends who run nicely unprotected computers, over the Internet
"net send ipaddress hiunprotectedpeople!"
you'll get a little windows popup message - great for getting my co-workers attention when they have headphones on - but fairly horrible to get porn spam grabbing your comps focus.
I run a basic but comprehensive personal software firewall on the @home network in Ontario, and can't seem to net send myself messages by ip.. I'm not blocking the port locally (port 143 or something near that) so I kind of expect my ISP is choosing to block it for me somewhere along the lines.
Spammers just send the same message to a fat range of ip addresses, and hope they hit.