Jon Udell points out why email is (almost) irreplaceable, whatever be the issues we have about spam and viruses:
Asynchronous messaging is one aspect of e-mail’s special power. It’s not the core benefit, though. If we peel another layer from the onion we find existing solutions that could be leveraged for store-and-forward messaging. Web-based forums, Wikis, and Weblogs are some of the messaging hubs that can enable groups to communicate independently of time and space. What they can’t support as easily or as effectively as e-mail is the dynamic formation (and dissolution) of those groups.
Every interpersonal e-mail message creates, or sustains, or alters the membership of a group. It happens so naturally that we don’t even think about it. When you’re writing a message to Sally, you cc: Joe and Beth. Joe adds Mark to the cc: list on his reply. You and Sally work for one department of your company, Joe for another, Beth is a customer, and Mark is an outside contractor. These subtle and spontaneous acts of group formation and adjustments of group membership are the source of e-mail’s special power.
Software that requires people to explicitly declare the formation of these groups, and to acknowledge their dissolution, is too blunt an instrument for such ephemeral social interaction. Like an operating-system thread, an e-mail thread is a lightweight construct, cheap to set up and tear down. Could a protocol other than SMTP, and an application other than e-mail, support such interaction? Sure, but any other communication medium that has e-mail’s special power to convene groups will suffer the same diseases that afflict e-mail: spam, abuse, infoglut.