In addition to statistics for publications and for individual accounts, you can now access stats for letters sent through your publication to your publication followers.
To begin, navigate to your publication's home page. Click on the down-arrow next to where it says either "Owner" or "Editor" (depending on your role) and from the drop-down menu that appears click on "Letters".

Click on "Letters" to access the letters dashboard. You will see a list of sent letters and their statistics below the the drafts section.

Notes
- As this is a new feature, Medium only has statistics for letters sent after Jan 6th, 2016. Any new letters you send will record their open rates and click through rates.
- The “Sent” column is the number of readers who were sent the email.
- The “Clicked” column is the number of readers who clicked on any link in the email. If you have that one reader who clicks on every link in the email, they’re only going to count as 1 user clicking.