He was POed because he didn't have a life, much less a truly decent ride like a Mercedes-Benz™. You looked like an easy target for his internalized bitterness and therefore, received his junk mail. At least he confronted you directly.
Around here, the bureaucracy has enabled loosers like that to have the state do their dirty work. The
TNRCC has an on-line reporting form where anybody, regardless of IQ or mental capacity can anonymously report you for excessive exhaust smoke whether or not the vehicle is actually violating the law. I'm currently in the process of collecting an entire set of these letters, one for each of my diesels.