http://newslamp.com/post/158516/home-made-robot-doing-a-perfect-quadruple-backflip

This is going out through my ISP (RCN), to stmp.rcn.com. I'm doing this from Canada, using the OS X mail client. When I try sending, the error message says:

    Sending the message content to the server failed.

    Select a different outgoing mail server from the list below 
    or click Try Later to leave the message in your Outbox until 
    it can be sent.

- If I modify the url after the domain name: fail

- If I change the "http" to "ahttp" (for example): fail

- If I remove "http://": fail

- If I change the domain name: email goes out

It's not an intermittent problem -- I've tried this about seven times by now, and it is completely reproducible.

What would explain this behavior? How can I figure out where the rejection is happening (mail client? mail server? Some server between here and there?)