Background sending. Background receiving. Very customizable, but worked well out of the box. I send email from my treo all the time. Versamail (the built-in treo email software) makes you watch its silly dialog box every time you send an email. If there was a problem (say you're underground in the parking garage) you have to figure out how to resend it later. At that point, I might as well hop on my pony and deliver the message myself. Chatter just seems to do what's right. As soon as you press "send", you can switch out of chattermail and look at your calendar or make a phone call. Chatter will get it done in the background. If it runs into a truly insurmountable problem, it'll pop up an alert. Chatter is so blackberry-cool, I've even started reading email on my treo, occasionally. Needless to say, all that background goodness for sending messages applies to receiving messages as well. And more.