You write documentation. You tell users to take issues and questions to the mailing list, submit issues or post in the public forums etc. Still you will find that lots of people will send private emails directly to key developers whose names and addresses people find in docs and email archives and ask questions or for favors.
People do this to avoid "the flood" of the mailing lists, by being naive about where issues should be taken and for trying to "take a shortcut" to ask the person they believe has the answers and would be the one who would answer the question on the mailing list anyway. There are also a fair share of "I do not want to air my problem or question in the public because of reasons."
Dealing with issues privately scales terribly. The questions are often similar and are much better taken care of in public so that many people can learn from the response and many people can help out to provide the responses in the first place.
I push back strongly on this behavior and either the user needs to take the issue to the proper public forum, or ask for a support contract setup to maintain the private discussion.