Most people selling by email make one of two mistakes, and the single post here names them both. You either give so much away that nobody ever buys, or you sell so relentlessly that nobody opens the message, and both feel like the safe option while you are doing them.
The third approach he describes satisfies both needs at once, and the claim attached to it is unusual. He says he pitches something almost every weekday, sometimes at weekends too, and receives almost no complaints for it, because of how the selling sits inside what people came for.
That is the whole blog. One article, close to a thousand words, on a subject most people get wrong for years before noticing.
If your list has gone quiet, or you have been apologising in every message before you get to the offer, this is worth the fifteen minutes. Read it before you write your next one, not afterwards.