There are no strict principles when it comes to newsletter marketing.
Sure, there are some good rules of thumb. But if they actually existed, then they’d stop being effective rules of thumb. Let’s say that based on its principles everyone sends newsletters at a specific time. In this case the marketing noise would be too high, and our newsletter couldn’t perform well.
This is why we do the following: we put some good ideas and promising directions into practice, test them (e.g. A/B testing), then draw conclusions and fine-tune if needed. The feedback loop ensures we are continuously improving newsletter by newsletter.