I have a general idea that there is a lot of broken software out there
gives HELO as a short, unqualified, non-existent or otherwise weird name.
I don't have a lot of data to back it up... I have just heard from others
in passing that HELO checking can be good, but currently requires a lot
white-listing and exceptions.
Who knows? Perhaps HELO checking and enforcement is incredibly important.
It could be. I just don't have any data to back that up.
A quick grep through my logs shows that of about 18% of HELOs have bad
syntax and 25% use a name with no DNS (sample 250k HELOs).
I believe Hotmail is pretty bad on the second point.