Pop ups are claimed to increase conversion rates of newsletter signups, and actually, they do.
But people hate them.
You may increase your newsletter signups from 0.1% to 1%, but you're going to REALLY anger 99% of the people visiting your site. This is a poor tradeoff in usability.
Search engines are acutely aware of this, and have been penalizing sites that have pop-ups/interstitials on the first visit to a website:
Adding a call to subscribe on exit is one potential solution.
You could also create a pop-up sign up for when a user intentionally clicks on a sign up button (not by tricking them into clicking anywhere), but this is unnecessarily complex. Better to just embed the sign up form right into the page itself.