Newton can notify you when someone opens an email you sent with read receipts. You get a notification on your phone and computer, and the desktop app keeps a list of opens in Pulse.
Steps
Desktop
Open Settings: press ⌘, on Mac, or Ctrl+, on Windows and Linux.
Go to the Supercharge tab, then Read Receipts.
Switch on Read receipt notifications.
To see past opens, click the Pulse bell next to Compose, or press N. Each open reads "Name opened your email". Click it to go to the email.
For an email you already sent, open it, click the double tick next to your message, and switch on Notify me when opened in the Read Status tab.
iPhone, iPad & Android
Open the menu and tap the Settings gear at the bottom.
Tap Supercharge, then Read receipts.
Switch on Read receipt notifications.
For an email you already sent, open it, tap the double tick next to your message, and switch on Notify me when opened in the Read receipts tab.
Good to know
The switch in Supercharge applies to emails you send after you turn it on. For earlier emails, use Notify me when opened.
You get one notification each time someone starts reading. Opens less than 10 minutes apart count as one.
The desktop app notifies you only while it's running. The Pulse list of opens is on desktop only for now. On your phone, tap an email's tick to see who opened it.
Some mail apps load images automatically, so you may be notified before anyone has read the email.