If the addressee replied "please don't write to me ever again", then writing again is rude. But I guess TS did not do that: there's no indication that he did, a

If the addressee replied "please don't write to me ever again", then writing again is rude. But I guess TS did not do that: there's no indication that he did, and even this "open letter" doesn't contain an explicit ask to stop mailing in the future. It's more likely that he replied with standard "thank you, I'm not interested in joining your company now" or quietly ignored the message. I personally find repeated messages from recruiters useful. Maybe I'm not interested in switching jobs now, but things may change in 3 months. Maybe a new position is opened. Maybe the salary range or compensation scheme is changed. Maybe I need more money because I got kids. Maybe I'm so desperate to find employment because of my visa status. Maybe I change my mind about the company. Maybe the company changes (like Microsoft changed in the last 5 years).