HubSpot has a service called Signals that enables you to see who opens your email.
Signals works with Gmail, Outlook for Windows, Salesforce, and Hubspot.
I really like it, because I am regularly working in an environment governed by deadlines, and it’s essential for me to know whether my messages are getting to my contacts and being read.
Since I can see where, when, and how many times an email has been opened/clicked, I know whether I need to follow up.
Being an email maniac, this calms me and helps my workflow.