Research has shown that adding variables in your subject line when addressing candidates significantly increases your response rate.
Variables for example, first name, last name, company name, full name, and location can be added in the subject line while sending emails to candidates.
Here is a quick snapshot for your reference:
I have taken my own name as an example for this case. As you can see in the screenshot below, I went ahead and added the "firstname" variable in the body and copied the same in the subject line.
Once you hit send, the email will look like this. You will notice my first name is picked from my email ID.