My policy is that if the email is legitimate and is well meant, I will reply in some fashion. I do not believe it is totally ethical to “blow off” another soul that wants to speak with you, unless that person has either been someone that is not known to you personally or has no professional interest or is threatening. That said, one should use the opportunity to set up boundaries of the experience and tell that other person your position on whether they have a place emailing you. If at the last resort they can not abide by your boundaries, you simply can edit them out and name them as junk mail. But that said, perhaps Hashem has a plan for each person that seeks to contact you and you might be missing a place in your world to be improving your relationship with Hashem through that other person. If they are clearly not sent by our Creator in an act of goodness, then it is clear that you should either not reply or you should reply in a sense that you discuss their infirmity or wickedness or what their resentment really is and you bid them “best wishes” in what they may be seeking to do further in their life. Fortunately, I must report that these messages are far and few in between and of course that is good. But that said, to simply blow off someone becuase they are the opposite gender sets a bad precidence in how you manage your friendships. True that it can be a squakey ground that they want to walk upon regarding a male-female relationship, but sometimes these can be professional or righteous personal relationships. In all, its your day that is to be decided by yourself and truly you are someone that must answer to Hashem and be happy with yourself. So if you think it is a pathway to a dark or licentious relationship, it is really upon you to be the strong being that will not let this happen to your soul. But a simple letter from an organized and well meaning person of the opposite gender is no reason to run into the corner and run away from your own self and run away from the day.
Thats my thought on this topic. I’m a doctor. Thats my policy.