At a guess: IE and apparently Firefox have "search for url" enabled by default. In IE that consists of sticking .com, .net, etc suffixes on, and I think trying a www. prefix. From a report on the user's list, it appears that Firefox goes farther and will do a google search, resulting in a tinyurl lookup if needed.
Now, whether that will make it into the email logic in OE, I don't know.
