Hello WiseSector,
I agree with
@McMcbrad's last statement "If they hate you, then you are doing something right."
If there is anything I've learnt, it is this: As soon as you become successful, some people feel threatened and will try to put you down. Why do they they feel threatened? Because they are not willing or able to put the dedication and hard work into becoming successful themselves. Putting you down is easier than doing the work. What you got in this email is an emotional outburst which means you really struck a nerve there.
Not sure if you want advice but since you posted here, I assume you wanted to get some. What usually might go through your head is "What did I do wrong? Is there truth to this email? Am I inediquate?". Forget about all of that. Put a mental stop to these thoughts. The truth the email is telling is not about you or your product but only about the sender.
See it as what it is:
1) It's a sign that you are successful. You will likely get more of these. But it's a good sign!
2) The author is the one who is inediquate and cannot deal with their own life or emotions.
You can ignore it or, if you feel like it, write something kind back. I am serious, because being kind to someone who attacks you, often stops them in their tracks. Your kindness should be truthful, though. Don't pretend.
