An email client might wrap the display of the URL onto a second line, but if it's worth it's salt it won't actually stick a newline in the string, which *would* wreck your URL (in the sense of making an unusable hypertext link, for clients that support that sort of thing).

Other than this mostly display-related issue, I can't think of what you might mean by a URL not fitting into an email. If it's aesthetics you're worried about, well, you already have that ugly string in there, so might as well overdo it while you're into it. (as Steve Vai would say).

So if you want to provide clickable links, try it with the full-dress md5 hash. An alternative thing is to truncate the hash and hope for the best, i.e. grab the last 16 characters and hope none of the values collides? Or maybe I shouldn't recommend anything so reckless ...

Philosophy can be made out of anything. Or less -- Jerry A. Fodor