Microsoft has become a spam emitter
I'll start by quoting my tweet:
I admire how Microsoft IP address space with no reverse DNS has now become a source of spam emitters using forged HELOs. Thanks, uSoft!
Let me show you the specific log entry that sparked this:
remote from [126.96.36.199] EHLO mail.rackspace.com 550 Unknown command 'EHLO' HELO mail.rackspace.com 250 [...] Hello mail.rackspace.com MAIL FROM:<firstname.lastname@example.org> 554 unacceptable from address: <email@example.com>
This IP address space is registered to Microsoft and has no reverse DNS. It is certainly not Rackspace. As it turns out this was probably a Windows Azure customer since this appears to be a Windows Azure datacenter range (in their 'useast' region). To determine this I had to dig up a Microsoft document on Azure Datacenter IP ranges from Internet searches.
I'm blaming Microsoft directly here because Microsoft consciously passed up the chance to clarify what the IP address was and who it might belong to. That's what reverse DNS is for, as shown by eg Amazon AWS (which gives their AWS IPs clear reverse DNS). Microsoft opted to keep their Azure IPs anonymous, so Microsoft gets to take the blame.
(Certainly as a sysadmin investigating a problem I'm not going to bother looking further when there is no functioning reverse DNS. Nor can I really do anything more precisely calibrated than acting on the entire registered netblock (not unless I want to pull that Microsoft data on a regular basis and examine it for changes).)
PS: The cynic in me is muttering that Microsoft decided to not do reverse DNS so that people like me couldn't just block based on the domain name being in whatever magic domain. I'm not sure I believe that, but it's certainly a tempting idea. (Anti-spam work makes one a cynic.)
PPS: This is apparently causing problems for Azure customers. I'm a little bit surprised that any large ISPs have started to reject email if you don't have reverse DNS; the last time I looked at this I assumed it was far too risky and not likely to be adopted by anyone major any time soon.