The Host Requirements RFC says that if you support "receiver SMTP", you must support the reserved mailbox "Postmaster".
In my experience, interpreting that statement has been a pre-existing exercise for the readers for nearly a decade, with many results.
The Host Requirements RFC also says, I believe, that you are to respond to PING -- try that with www.microsoft.com. My point is, it appears that the Host Requirements RFC seems to be "interpreted" in the real world. Lots of sites seem to not implement postmaster. But then again, sites seem to not maintain contact info in the whois database that's valid, and I would think that would impact network operations even more... Is there a BCP-like list somewhere? Or has this, like other things, been discusssed to death on NANOG in decades gone by...