diff --git a/bramble-core/src/main/java/org/briarproject/bramble/mailbox/ContactMailboxClient.java b/bramble-core/src/main/java/org/briarproject/bramble/mailbox/ContactMailboxClient.java
new file mode 100644
index 000000000..35e0f0d4f
--- /dev/null
+++ b/bramble-core/src/main/java/org/briarproject/bramble/mailbox/ContactMailboxClient.java
@@ -0,0 +1,109 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+import javax.inject.Inject;
+
+@ThreadSafe
+@NotNullByDefault
+class ContactMailboxClient implements MailboxClient {
+
+	private final MailboxWorkerFactory workerFactory;
+	private final ConnectivityChecker connectivityChecker;
+	private final TorReachabilityMonitor reachabilityMonitor;
+	private final Object lock = new Object();
+
+	@GuardedBy("lock")
+	@Nullable
+	private MailboxWorker uploadWorker = null, downloadWorker = null;
+
+	@Inject
+	ContactMailboxClient(MailboxWorkerFactory workerFactory,
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor reachabilityMonitor) {
+		this.workerFactory = workerFactory;
+		this.connectivityChecker = connectivityChecker;
+		this.reachabilityMonitor = reachabilityMonitor;
+	}
+
+	@Override
+	public void start() {
+		// Nothing to do until contact is assigned
+	}
+
+	@Override
+	public void destroy() {
+		MailboxWorker uploadWorker, downloadWorker;
+		synchronized (lock) {
+			uploadWorker = this.uploadWorker;
+			this.uploadWorker = null;
+			downloadWorker = this.downloadWorker;
+			this.downloadWorker = null;
+		}
+		if (uploadWorker != null) uploadWorker.destroy();
+		if (downloadWorker != null) downloadWorker.destroy();
+	}
+
+	@Override
+	public void assignContactForUpload(ContactId contactId,
+			MailboxProperties properties, MailboxFolderId folderId) {
+		if (properties.isOwner()) throw new IllegalArgumentException();
+		// For a contact's mailbox we should always be uploading to the outbox
+		// assigned to us by the contact
+		if (!folderId.equals(properties.getOutboxId())) {
+			throw new IllegalArgumentException();
+		}
+		MailboxWorker uploadWorker = workerFactory.createUploadWorker(
+				connectivityChecker, properties, folderId, contactId);
+		synchronized (lock) {
+			if (this.uploadWorker != null) throw new IllegalStateException();
+			this.uploadWorker = uploadWorker;
+		}
+		uploadWorker.start();
+	}
+
+	@Override
+	public void deassignContactForUpload(ContactId contactId) {
+		MailboxWorker uploadWorker;
+		synchronized (lock) {
+			uploadWorker = this.uploadWorker;
+			this.uploadWorker = null;
+		}
+		if (uploadWorker != null) uploadWorker.destroy();
+	}
+
+	@Override
+	public void assignContactForDownload(ContactId contactId,
+			MailboxProperties properties, MailboxFolderId folderId) {
+		if (properties.isOwner()) throw new IllegalArgumentException();
+		// For a contact's mailbox we should always be downloading from the
+		// inbox assigned to us by the contact
+		if (!folderId.equals(properties.getInboxId())) {
+			throw new IllegalArgumentException();
+		}
+		MailboxWorker downloadWorker =
+				workerFactory.createDownloadWorkerForContactMailbox(
+						connectivityChecker, reachabilityMonitor, properties);
+		synchronized (lock) {
+			if (this.downloadWorker != null) throw new IllegalStateException();
+			this.downloadWorker = downloadWorker;
+		}
+		downloadWorker.start();
+	}
+
+	@Override
+	public void deassignContactForDownload(ContactId contactId) {
+		MailboxWorker downloadWorker;
+		synchronized (lock) {
+			downloadWorker = this.downloadWorker;
+			this.downloadWorker = null;
+		}
+		if (downloadWorker != null) downloadWorker.destroy();
+	}
+}
diff --git a/bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxClient.java b/bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxClient.java
new file mode 100644
index 000000000..8dc4ab4e1
--- /dev/null
+++ b/bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxClient.java
@@ -0,0 +1,46 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+interface MailboxClient {
+
+	/**
+	 * Asynchronously starts the client.
+	 */
+	void start();
+
+	/**
+	 * Destroys the client and its workers, cancelling any pending tasks or
+	 * retries.
+	 */
+	void destroy();
+
+	/**
+	 * Assigns a contact to the client for upload.
+	 */
+	void assignContactForUpload(ContactId c, MailboxProperties properties,
+			MailboxFolderId folderId);
+
+	/**
+	 * Deassigns a contact from the client for upload.
+	 */
+	void deassignContactForUpload(ContactId c);
+
+	/**
+	 * Assigns a contact to the client for download.
+	 */
+	void assignContactForDownload(ContactId c, MailboxProperties properties,
+			MailboxFolderId folderId);
+
+	/**
+	 * Deassigns a contact from the client for download.
+	 */
+	void deassignContactForDownload(ContactId c);
+}