You can think of
Email Marketing this way: with every group of individuals you persuade to join your email list, you get interested visitors who could potentially become leads for you.
A large part of Email Marketing is sending promotional emails about service updates, increasing sales, or simply keeping people informed about everything your brand has to offer. Email marketing has changed dramatically as a result of changing trends.
Assume you're in your late twenties, settling into your careers, and you receive two promotional emails, one about undergrad tutorials and the other about investing and building wealth.
Which one will pique your interest? Obviously, the latter since it corresponds to your current interests, whereas the former is as useless as trash to you. Isn't that so?
This is why, at least for the time being, you can't use a one-size-fits-all technique. Consumers aren't interested in learning about services they don't desire or will never use.
I hope you understand the point that I discussed here. You can read this article to know a deep answer of your Question and strategies to do email marketing:
Email Marketing: Budding Trends