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When composing an email in Mailspring, check these icons in the composer to enable read receipts and link tracking. That's all you need to do!
When your emails are read or links are clicked, you'll see notifications in the top left of the main window. You'll also see icons and tags in your Sent folder, so you can get more information at a glance.
For even more detail, go to your Sent folder and view the email - clicking the purple "eye" icon allows you to see a list of everyone who opened the email, and clicking tracked links allows you to see who clicked them:
If you enable read receipts and link tracking on many of your emails, Mailspring's account activity view is a powerful tool to assess how your emails are performing. You can read more about that feature here.
By default, Mailspring will notify you each time one of your tracked emails is viewed or a link is clicked. If you'd rather only receive a notification the first time an email is viewed, check out the options in Preferences > General.