Most email clients highlight such URLs and clicking on such a link will open the address in the user's browser. They are looking for "http://" or "ftp://" in the text of the message and assume everything immediately following this up to but not including the first white space or return belongs to the URL.
Be Careful with Punctuation Around URLs
This is why you should be careful with punctuation around URLs and not write an address like this:
More about FooBar Inc. at http://email.about.com.
Instead, you could put the period after a white space character:
More about FooBar Inc. at http://email.about.com .
or surround the URL with less than and greater than signs, which never belong to a URL but delimit it:
More about FooBar Inc. at <http://email.about.com>.