Email rejected on sending due to content?

This is bizarre. I am trying to send an email containing this link: 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 change the email message by adding text: fail - 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?)