What types of email verification techniques are used by email checkers?

I'm exploring different email checker tools and I'm curious about the various techniques they use to verify email addresses. Understanding these methods will help me choose the most effective solution for my needs. Could anyone explain the different types of email verification techniques commonly employed by email checkers? Specifically, I'm interested in how these techniques work, their accuracy, and any pros and cons associated with them. Insights from professionals who have experience with these tools would be incredibly helpful!