Who would write an email server designed to serve thousands of addresses that included a guest account. Who would be so stupid? That would be Microsoft.
Apparently the guest account hole isn’t all that common. Microsoft is claiming that they don’t hear a lot of reports from people whose Exchange servers have been exploited in this way.
Small wonder. Few sysadmins would want to admit to being so stupid as to leave this account enabled, even though far more idiotic is the company that creates the server install with a guest account in the first place.