[mailop] Try to understand *.onmicrosoft.com

[MRob via mailop]

Is envelope sender user@<anything>.onmicrosoft.com normal in non-spam 
mail? Is it how all microsoft mail comes through? Or is it usually spam 
from badly configured domain? Should <anything> part *always* match 
sender domain in FROM header?

On the other hand, if mail come from microsoft server *not* through 
"onmicrosoft.com" is that negative sign?

Thank you.
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