Barbara,
You should to a View Source on an HTML email, the real mail content is supposed to have both an HTML and a plaintext version of the email inside it. That way older email programs still have something displayable.
Other than that you might put words to the effect warning people that the URL may be chopped off. It's also possible that the chopping will occur at their receive end and not necessarily on your send end.
>That's a good idea - I'll use it as a last resort. Since these messages will be going world-wide to people with every imaginable internet connection and email provider I'd really like to keep it as simple as possible.