Paste the subject and the body. Ten checks run as you type: length, reading time, words that trip spam filters, links, how much the email talks about the reader, and whether it asks one clear question. Runs in your browser.
Short (under 120 words), about the reader more than the sender, one specific reason for writing now, and one easy question at the end. No images, at most one link, no attachment on the first touch.
One rarely sinks an email on its own; a cluster of them with links and images does. The checker flags them so the choice is yours.
No. Every check runs in your browser and nothing leaves the page.
Guides for the same job, run from your agent