Along with reports from professional astronomers, which come in daily, Marsden hears from lots of amateurs who think they've found a new supernova or comet, which more often than not turns out to be a known object, or an artifact of the detector, or nothing at all. The man had made a simple observing error, and finally Marsden asked a Danish astronomer friend to explain to the observer what his mistake was. Mostly, though, Marsden receives routine reports of comets, novas, and asteroids and follows up information on orbital parameters and the like. He waits for confirmation, and if the report checks out, he sends a notice to the rest of the astronomical community. The IAU circulars have traditionally gone out as postcards, but now many people get them by e-mail.
Most astronomers report their discoveries that way too.
Marsden doesn't recommend faxing, because he and his two colleagues at the Bureau don't check their machine regularly enough. Ditto the phone. More importantly, e-mail messages don't need to be retyped, saving time and probably mistakes. Given the unpredictability and suddenness of astronomical events, you'd think Marsden would be continually getting yanked out of bed at all hours. He can think of only one case where it would have been warranted.