I'm using this tool built into gmail as a chrome plugin. Its main features are:
- let you see when someone reads your email via browser notification
- see email read history (dates & times)
- let you write email templates for canned responses
- lets you send a public web link to an email thread
- has pipelines for managing projects (called boxes) like a simple crm
- lets you add fields that you can tag to projects
- some team collab stuff
It's a freemium model, but the free version has plenty and I've used it for 3+ years with no issues. The paid options are for mail merge and team stuff.
Mod: Removed referral link.
So tracking pixel software/plugin then?
Ugly email plugin fixes that up :)