Well, none of the client-side mail clients that I've used (Lookout, outlook express, netscape, eudora, pegasus) pop up a window saying the message was sent successfully either. If there's an error in sending the message (like you haven't entered a recipient e-mail address), IMP 3.x will tell you. If you've entered an invalid address, you'll get a bounce (which is all you'd get with 2.2.x).
I can't address the question of what the users on your LAN want, but I disagree that cluttering the screen with information that should be obvious (should be obvious because no error was displayed) is a more logical route to take. Software is about serving humans, but different humans want different things.
I don't know of any way of adding this functionality to IMP 3.x. I'm sure it's possible, but it may involve writing some new code. Your best bet is probably to check with the horde project at
www.horde.org.