There is only one real rule: Get their attention.The point of the query is to sell your book, make people want to read it (make an agent think it's worth their time, energy, and essentially money in reading your novel)
Everything else is mere guidelines, suggestions, for how to structure your query. Believe me, I've been reading everything I can find on query's. It's exhausting and confusing.
No lie, one agent said "Don't start with a question, it's annoying and distracting and what if my answer is no?" The next blog said, "Starting with a rhetorical question is good hook to get agents interested."
Just like everything else in life, it's all subjective. There are no hard and fast rules. And even if there were, it's the innovators who typically get noticed, and they tend to think outside the box. Make their own way. There are no rules: See this post.
Now with all that said, here are a few guidelines that just plain make sense:
- Keep it under 250 words (agents have limited time, you have to hook them quickly)
- Include all contact information in closing.
- FOLLOW THE AGENTS RULES (they usually have their own guidelines to submitting.)