How I respond
I am sorry to say that I usually do not respond at all.
The distance in technical knowledge and awareness of how things are glued together and work between the person sending the email and me is usually so great I do not know how to overcome it.
Years ago I used to reply and try to explain for the other side that I am but the main author of an Open Source library one of these giant corporations are using, but it backfired so many times I have realized it often just makes the user more upset than before. They often do not believe me. Since my code is clearly used in their product I must know the company, so when I do not help out the person with the problem, I get even more in the receiving end of their desperation. They end up believing I am trying to duck for or avoid the problem when I should help them.
I do occasional exceptions, like if the user seem to be the victim of a scam or is in a truly vulnerable situation.