As a professor writing to another professor, should I use 〜さん (like in other workplaces) or 〜先生?
Since you're writing to a teacher, you must stick to 先生. This is the same even if you are a university president, a prime minister or a Nobel Prize winner. Don't use さん unless they have become your personal friend.
As a professor writing to a junior staff member (doctoral student or post-doc), what is the right form of address?
- If that staff member holds a title as a teacher (e.g., 教諭, 助教, 講師, and of course, 教授), always use 先生.
- If that staff member holds a license such as a medical license, which warrants being called 先生 even by the general public, then you use 先生, too.
- For grad students, exchange students, post-doc researchers (paid or unpaid), and research assistants who do not hold such titles or licenses, you use さん when writing to your subordinates, and 様 when writing to those outside your research group. It's also generally safe to use さん when addressing such outside researchers in direct conversations.
I have noticed in some emails that another professor addresses the junior researcher as 〜君, but I am wondering if that is a general thing.
君 is still commonly used in some universities (慶應義塾's convention is well-known), but it's generally considered outdated in business and academic settings.