What is the legitimate legal control in the matter of ensuring safety from temptation when a man addresses a woman or vice versa? And does it pertain to mahrams rather than non-mahrams, or to age, or to beauty, or to the state of the times? And what is the ruling on addressing now?

"Fitnah" (tribulation/temptation) is a test or an examination. In common usage, it has been used to uncover what is disliked. The Prophet, peace and blessings be upon him, informed us that the fitnah of women is more harmful to men. To avoid this fitnah, Islamic law has forbidden private seclusion with a non-mahram woman (khalwa), prohibited women from speaking in a soft and alluring manner, and commanded lowering the gaze. Everything from which fitnah is feared is analogized to these prohibitions, such as returning a greeting or saying "Yarhamukallah" (May Allah have mercy on you) to someone who sneezes if it leads to fitnah, regardless of the age or beauty of the individuals or the corruption of the times. Therefore, the wise person should keep away from places of suspicion.