Statistically only 80% of your mailing list will actually see the email subject. The percent of people opening that email largely depends on the subject line. The more targeted your audience is, the better chances you can create a subject line that will cause them to open the email. If you don't know who your audience is or haven't put a lot of time in figuring out what subject lines will work, then you won't have a good opening %.
Your first emails always need to be A/B tests to see what subject lines work. Keep the target audiences and email bodies the same. When you find a subject line that gets better open %, do another set of samples for email intros, bodies, conclusions, and CTAs.
Send out mass emailing campaign after seeing acceptable results from your samples.
Edit: Adding on to this... To earn an open, build curiosity. Curiosity is a gap of knowledge between knowing something and wanting to know more. In the body, it's a good idea to use conversational layering. This is an attention grabber->analytical/logical speech to build credibility (showing them you know about them, their job, their problems)->dig into pain points and make it more emotional->ask "wouldn't it be great if you had a solution" (bait them with a solution, but don't give it to them or they won't have a reason to click)->You would solve A, B, C paint points and get X, Y, Z benefits->CTA