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docs/DESIGN.md

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+# Repository structure
+
+The project is divided into several directories below which are nested gradle projects.
+
+## app
+
+This contains the highest level code such as UI and core logic.
+
+## backend
+
+APIs for sending and receiving messages
+
+## mail
+
+Low level code for dealing with internet mail protocols
+
+## plugins
+
+Additional, standalone, libraries used by K-9
+
+![modules](Modules.png)
+
+# Walkthrough
+
+To help you understand the design, the following sequence diagrams show typical flows through the
+classes. Each class is colour-coded by its top-level project.
+
+## Reading email
+
+![read email sequence](ReadEmail.png)
+
+![read email classes](ReadEmailClasses.png)
+
+## Sending email
+
+![send email sequence](SendEmail.png)