This section of the documentation covers information for developers.

Getting Started

To develop K-9 you need a working understanding of:

  • Java
  • XML
  • Android

With just Java knowledge you may be able to make minor logic changes and small fixes. But the Android APIs are used extensively (as well as Android based libraries), particularly in the front-end.

XML provides the translation (however that’s pulled in from Transifex anyway) and some of UI layout.

Once you’ve gained a solid understanding of these we recommend you look at our existing issues and pick one tagged ‘good first issue’. If there’s not a tagged one not being worked on then feel free to pick an issue that interests you. If there’s no developer comments on the issue you select (and it’s not marked beginner) feel free to ask about the best approach before you start!

To build the project simply clone the repository and import the project into Android Studio. From there you can use the IDE tools to build the k9mail APK and install it.


The general structure of K-9 is as follows:


  • k9mail - Main module - includes code for activities, notification, database interaction, settings and encryption
  • k9mail-library - Back-end code for contacting mail providers and decoding emails from MIME
  • Android-PullToRefresh library - Library for implementing ‘pull to refresh’ lists
  • openpgp-api - Library for interacting with OpenPGP providers (e.g. OpenKeychain)
  • HoloColorPicker - A colour picker for the Holo Android theme

Core Design

There are many parts to K-9. This list gives a rough outline of some of them.

  • Stores (*) - Each receiving protocol (IMAP, POP3 and WebDAV) is implemented as a store (e.g. ImapStore). This store maintains connections (e.g. ImapConnection), and provide access to Folders (e.g. ImapFolder). which correspond to email folders that store messages (ImapMessage).
  • Transports (.mail.transport.*) - Each sending protocol (SMTP, WebDAV) is implemented as Transport that must establish a connection to send e-mail.
  • Messages (.mail.Message, .mail.internet.*) - A message is comprised of Parts. The type of part actually used depends on whether the message is a RemoteMessage, waiting for download, mid-download (MemoryParts) or stored in the database (LocalPart).
  • MessagingController (.controller.MessagingController) - This key class is responsible for syncing and handling user actions. For example, it has methods to queue synchronization of messages.
  • Activities and Fragments (.activity , .fragments) - These back the various layout files to react to and display information to the user.
  • Notifications (.notification) - This comprises the Android notification handling

Activity Layouts

This section highlights some of the layout files in use.

  • account_setup_* - Views relating to account setup
  • accounts - The Accounts view
  • message_list - Renders the message list and view. Delegates to message_list_fragment and message depending on settings and requirements.
  • message_list_fragment - Renders the message list (a ListView of message_list_items)
  • message - Renders a message, delegating to message_view_header and message_container
  • message_compose - Renders the screen when composing a message. Delegates to message_compose_recipients and provides a list of message_compose_attachments. Uses a combination of enhanced EditText and MessageWebView for the message to be sent
  • message_container - Renders a message content, delegating to MessageWebView and displaying message_view_attachments
  • message_list_item - Displays a summary of a message for the list view.

Scheduling & System Intents

K-9 currently uses the AlarmManager API for scheduling tasks related to fetching email


  • K9AlarmManager provides a slight abstraction over the AlarmManager to use setAndAllowWhereIdle where available
  • BootReceiver and CoreReceiver are responsible for receiving Intents from Android
  • BootReceiver schedules and receives scheduled intents with the K9AlarmManager
  • The MailService dispatches Intents to the BootReceiver to reschedule polls and refresh push receipt connections
  • To create IMAP PUSH email connections, the MailService service starts the PushService and then instructs the MessagingController to start pushing
  • The PushService does very little. It’s purpose is to ensure that K-9 remains running while push connections are established
  • The PollService performs a poll on the given account - again done as a service to ensure K-9 remains running during this period, regardless of whether UI is visible.
  • The RemoteControlService also dispatches Intents to refresh pushing and polling as demanded by the remote application.
  • The SleepService is used to delay push reconnection on error.


  • SCHEDULE_INTENT - Indicates an application wants the given ALARMED_INTENT extra to occur AT_TIME in the future
  • FIRE_INTENT - Dispatched by the K9AlarmManager when a scheduled ALARMED_INTENT should happen
  • CANCEL_INTENT - Cancels a previously scheduled ALARMED_INTENT from firing.


Each LocalStore has a separate LockableDatabase which controls access to an SQLiteDatabase. Hence querying multiple accounts requires accessing separate database files. The database is stored by a StorageProvider (of which there are several types with specific support for Samsung Galaxy and HTC devices).


  • folders - Contains the list of folders (and caches various parameters like unread count)
  • messages - Contains the messages. Has fields to store useful fields like preview, text, subject. References it’s root message part’s id (message_part_id).
  • message_parts - Contains a MIME message part
  • threads - Stores a message_id, it’s parent message_id and root message_id.
  • pending_commands - The queued outstanding commands requiring processing.
  • Virtual Table: messages_fulltext - Used for searching message contents

Third Party Integration

K-9 is built on Android, as such it uses two main methods of providing an interface for external applications: