When is an academic a -sensei and when is he/she a -hakase?
I've lived in Japan for two years and can speak the language to some extent.
Keigo is complicated, and any rule you learn about seems to have lots of exceptions. After two years, I felt that I had some idea, but I felt that I had far from mastered the intricacies. In particular, older people often addressed me using keigo for reasons I didn't quite understand.
&quot;Sensei&quot; is a general respectful term for a teacher. Since he is interested in an academic collaboration specifically, perhaps he regards you as &quot;senior&quot; because you have an academic position and he does not.
&quot;Sensei&quot; is a general and common form of address, used for academics and other teachers. I have not heard &quot;hakase&quot; used as a form of address.
In any case, he has certainly addressed you with respect. If you're unsure of what Japanese title to use, and your correspondence is in English, I'd encourage addressing him in English as &quot;Mr. X&quot; or (if he has a doctorate) &quot;Dr. X&quot;.