It generates an email address that you send your email to. Then you click a button and it tells you the spam score, and also specifically what rules you triggered and how badly each issue hurt you. Then you can edit your message to avoid those issues.
Not sending from a freemail account helps with delivery rates as well, so instead set up email on your own domain and create the proper DKIM/SPF records.
But the best way to avoid spam filters is to not spam, only send a very limited number of custom tailored messages. I also keep the initial message really short to avoid flags, once the other party replies you can generally say anything you want without worrying about spam filters because now you're on their contact list.