A recently added answer on SE.Robotics was from an engineer from a company, offering help with the product referred to in the question. They disclosed their affiliation and a link provided was for a course that they offered. This seemed to be above board, according to the spam rules, although you do have to pay for the course.
However, they added, what I presume to be, their support email address, which I edited out.
But it made me wonder, if the link to the course is allowed, then why shouldn't their email address?
I have read both What to do if user inadvertently includes email in a post? and What should I do if a user posts sensitive information as part of a question or answer? and neither seem to answer this question, as the email was neither inadvertently entered, nor does it seem particularly sensitive.