Neighbor threatens legal action for tenants behavior.


A neighbor (which is a business) of a property I own and lease out (to 3 different tenants) is complaining that my tenants are trespassing, damaging property, and burning illegally. They have made this complaint before that my tenants vehicles, etc, are flowing over the property line. It looks to me as that is not the case but I have never had it surveyed, so I don’t know where the lines really are, but I don’t know what I can do since I don’t know where the lines are. Last month my tenants burn barrel got away from them and the fire department came and put out a brush fire, the neighbor claims it was on their land, I don’t know. My tenants were cited (according to my neighbors.)
Now my neighbor sends me a certified mail outlining the issues and says “You are hereby notified of this second violation regarding trespass and property damage inflicted by your tenants.” and that they want to sit down and tell me how I’m going to resolve the issue (which I presume is to evict my tenants who haven’t missed a payment in 3 years) and "Failure to respond to this letter in a timely manner will result in … pursuing legal action to obtain appropriate remedies."
I’m hoping that this isn’t really my problem if it’s a trespass issue how am I to stop my tenants from trespassing if the neighbor doesn’t even survey the line? Plus I’m upset already with this neighbor as they called the local government to come inspect out “raw sewage overflowing” which turned out to be chicken poop getting rained on, that one cost me hundreds, and a lot of time getting it straightened out. Anyway, do I need to respond to these folks or is this between them and my tenants? Thanks.