I am currently chasing a couple of late-paying clients and I have looked for an email tracker to see if the other person is seeing my emails and decide to ignore them, or if they end up in their spam folder and aren't even looked at.
First result on Google was https://mailsuite.com/ which I installed as a Google Chrome add-on. I then accessed my Gmail account, authorized the extension to access it, sent two not-so-friendly reminder of overdue payment... et voilá. In 15' both had read my email.
I hope they will reply and MOREOVER send their payment.