Hotmail seems to have done something in the last 3 weeks. I get a lot of these bounces but those are actually sourced from some spam
website hostings. Someone tries to send messages to
xxyy @hotmail.com from aabbcc@ somecorporatedomain.com and hotmail bounces it. It appears that earlier Hotmail was accepting mail from servers which did not have enough information in the header to authenticate the sender hostname/IP but now after stricter security implementations, those without proper information are rejected.
Surprisingly, they send a 550 - which means user does not exist.
Any light on this would be appreciated.
Regds
IJ