From Macworld (snipped for length):
If you send URLs using your e-mail program, you may have an issue where the receiving client breaks really long URLs. For instance, if you were trying to send a URL for a site like this one:
http://www.amazon.com/gp/product/B000BI5KV0/ ref=amb_link_4263902_/002-5510588-1062447
You might find that your recipients are unable to click on the link in their e-mail program. That’s because many e-mail programs will do what I did above – break really long “words” (for that’s all a URL is to them) into multiple rows. Typically when this happens, only part of the URL will show up as a clickable link. When this happens, your recipients will get a “page not found” message when they try to load the URL – because the browser will only receive that portion of the URL that showed as clickable.
The simple thing to do is to surround your URL with angle brackets, like this:
<http://www.amazon.com/gp/product/B000BI5KV0/ref=amb_link_4263902_/002-5510588-1062447>
URLs enclosed in angle brackets should still be clickable – even if the e-mail client inserts a line break for formatting purposes. In 1998, the Network Working Group of the Internet Engineering Task Force (IETF) wrote a generic syntax for Uniform Resource Identifiers. The working group describes how URIs enclosed in angle brackets should be parsed when the use of white space (such as line breaks) is required for formatting purposes. Any e-mail client that follows these guidelines should display a clickable link, even if the line is wrapped.
I never knew this! 🙂