I'm not Japanese, and I've been emailed by someone who is Japanese, in the context of potential academic collaboration. He's in industry, not academia. He used the "-sensei" honorific to refer to me, while I had called him using his company-position-related honorific.
This seemed strange to me, since he's older and more senior than me (in most respects anyway). I was assuming he'd call me "-san", or if he wanted to be nice, then "-hakase" or no honorific at all.
My question: In what circumstances is it customary to call non-medical academics, with a doctoral degree, "-sensei", and in what circumstances is "-hakase" the custom?