It's a bit of a shame that Is "pal" too informal when the other person is much older than me? was closed, as it dabbles in a difficult topic for all non-native speakers of English. Although I realise that this question, too, might be closed quickly for its open nature, I'd still like to try and find a way to put it in a useful manner.
As a non-native speaker of English, I often am in a situation where it is unclear to me how to best address a person older or in higher social standing than myself. In spoken conversation it is easy to circumnavigate the issue, but in written communication, the requirement for opening ("Dear...") and closing ("Best regards, ...") lines makes this much more difficult. As such, I would like to limit the scope of this question to written communication.
In short, and divided between BE and AE: what are the expectations of an older or higher-ranked (i.e. someone with a PHD title) person when being addressed by a younger or lower-ranked (i.e. someone without an academic title) person? And also, again divided by BE and AE: what are usual markers that might be employed by the older or higher-ranked person to indicate that they would prefer a lower level of formality, say, a drop from a last-name basis down to a first-name basis?
If the community feels that this question, too, should be closed, it would be helpful to get suggestions as to what changes might make it viable for English Language & Usage.