diff --git a/packages/main/src/cache/persist.ts b/packages/main/src/cache/persist.ts
index 8f5f5a7e..f874b295 100644
--- a/packages/main/src/cache/persist.ts
+++ b/packages/main/src/cache/persist.ts
@@ -273,12 +273,41 @@ export function addFlag({
       join boxes on box_id = boxes.id
       where
         messages.account_id = @accountId
-        and messages.uid in (${uids.join(", ")})
         and boxes.name = @boxName
+        and messages.uid in (${uids.join(", ")})
     `
   ).run({ accountId, boxName: box.name, flag })
 }
 
+// TODO: What is the proper way to provide a list of values in a query?
+export function delFlags({
+  accountId,
+  box,
+  uids,
+  flags
+}: {
+  accountId: ID
+  box: { name: string }
+  uids: number[]
+  flags: string[]
+}) {
+  db.prepare(
+    `
+      delete from message_flags
+      where
+        message_id in (
+          select messages.id from messages
+          join boxes on box_id = boxes.id
+          where
+            messages.account_id = @accountId
+            and boxes.name = @boxName
+            and uid in (${uids.join(", ")})
+        )
+        and flag in (${flags.map(f => `'${f}'`).join(", ")})
+    `
+  ).run({ accountId, boxName: box.name })
+}
+
 function insertInto(table: string, values: Record<string, unknown>): ID {
   const keys = Object.keys(values)
   const { lastInsertRowid } = db
diff --git a/packages/main/src/queue/better-queue-better-sqlite3.ts b/packages/main/src/queue/better-queue-better-sqlite3.ts
index 53aa8276..ca475817 100644
--- a/packages/main/src/queue/better-queue-better-sqlite3.ts
+++ b/packages/main/src/queue/better-queue-better-sqlite3.ts
@@ -58,7 +58,7 @@ export default class SqliteStore<T> implements BetterQueue.Store<T> {
         )
         .get({ taskId, lock: "" })
       if (row == null) {
-        return null
+        return undefined // Failure result must be `undefined`, not `null`!
       }
       return JSON.parse(row.task)
     })
diff --git a/packages/main/src/queue/index.test.ts b/packages/main/src/queue/index.test.ts
index c50a5fb0..226af753 100644
--- a/packages/main/src/queue/index.test.ts
+++ b/packages/main/src/queue/index.test.ts
@@ -64,7 +64,80 @@ it("marks a conversation as read", async () => {
   )
 })
 
-it.skip("replaces pending read status change when a new change is queued", async () => {})
+it("marks a conversation as unread", async () => {
+  const connectionManager = mockConnection()
+  await sync(accountId, connectionManager)
+
+  const promise = queue
+    .enqueue(
+      queue.actions.unmarkAsRead({
+        accountId: String(accountId),
+        box: inbox,
+        uids: [7687]
+      })
+    )
+    .toPromise()
+  const flags = db
+    .prepare(
+      `
+        select flag from message_flags
+        join messages on message_id = messages.id
+        where
+          uid = @uid
+      `
+    )
+    .all({ uid: 7687 })
+  expect(flags).toEqual([])
+  await promise
+  expect(Connection.prototype.delFlags).toHaveBeenCalledWith(
+    [7687],
+    ["\\Seen"],
+    expect.any(Function)
+  )
+})
+
+it.skip("replaces pending read status change when a new change is queued", async () => {
+  // pause queue
+  const connectionManager = AccountManager.connectionManagers[String(accountId)]
+  delete AccountManager.connectionManagers[String(accountId)]
+
+  const promise1 = queue
+    .enqueue(
+      queue.actions.markAsRead({
+        accountId: String(accountId),
+        box: inbox,
+        uids: [7687]
+      })
+    )
+    .toPromise()
+  const promise2 = queue
+    .enqueue(
+      queue.actions.unmarkAsRead({
+        accountId: String(accountId),
+        box: inbox,
+        uids: [7687]
+      })
+    )
+    .toPromise()
+
+  // resume queue
+  AccountManager.connectionManagers[String(accountId)] = connectionManager
+
+  queue.queue.resume()
+  const flags = db
+    .prepare(
+      `
+        select flag from message_flags
+        join messages on message_id = messages.id
+        where
+          uid = @uid
+      `
+    )
+    .all({ uid: 7687 })
+  expect(flags).toEqual([])
+  await Promise.all([promise1, promise2])
+  expect(Connection.prototype.addFlags).not.toHaveBeenCalled()
+})
 
 afterEach(() => {
   db.prepare("delete from accounts").run()
diff --git a/packages/main/src/queue/index.ts b/packages/main/src/queue/index.ts
index d0eb485d..8f16b48d 100644
--- a/packages/main/src/queue/index.ts
+++ b/packages/main/src/queue/index.ts
@@ -34,16 +34,37 @@ export const { actions, actionTypes, perform } = combineHandlers({
         ])
       )
     )
+  },
+
+  unmarkAsRead(
+    _context: unknown,
+    {
+      accountId,
+      box,
+      uids
+    }: { accountId: ID; box: { name: string }; uids: number[] }
+  ): R<void> {
+    return withConnectionManager(accountId, connectionManager =>
+      connectionManager.request(
+        request.actions.delFlags({ name: box.name, readonly: false }, uids, [
+          "\\Seen"
+        ])
+      )
+    )
   }
 })
 
 type Task = ActionTypes<typeof actions>
 
-export function enqueue<T extends Task>(action: T): R<ActionResult<T>> {
+export function enqueue(action: Task): R<ActionResult<typeof action>> {
   if (action.type === actionTypes.markAsRead) {
     cache.addFlag({ ...payload(action), flag: "\\Seen" })
+  } else if (action.type === actionTypes.unmarkAsRead) {
+    cache.delFlags({ ...payload(action), flags: ["\\Seen"] })
   }
-  const result = promises.lift1<ActionResult<T>>(cb => queue.push(action, cb))
+  const result = promises.lift1<ActionResult<typeof action>>(cb =>
+    queue.push(action, cb)
+  )
   return kefir.fromPromise(result)
 }
 
@@ -66,7 +87,7 @@ function processTask(
   cb: ((error: Error) => void) &
     ((error: null, result: ActionResult<typeof task>) => void)
 ) {
-  perform(undefined, task).observe({
+  perform(1, task).observe({
     value(v) {
       cb(null, v)
     },
@@ -86,11 +107,15 @@ const store =
         path: getDbPath()
       })
 
-const queue = new BetterQueue<Task>(processTask, {
-  maxRetries: 3,
-  maxTimeout: 5000,
-  retryDelay: 5000,
-  store
+export const queue = new BetterQueue<Task>(processTask, {
+  maxRetries: Infinity, // keep retrying until we come online
+  maxTimeout: process.env.NODE_ENV === "test" ? 150 : 10000,
+  retryDelay: process.env.NODE_ENV === "test" ? 1 : 10000,
+  store,
+  merge(_oldTask, newTask, cb) {
+    console.log("merge", _oldTask, newTask)
+    cb(null, newTask)
+  }
 })
 
 export function getQueuedTasks(): Array<Action<unknown>> {
diff --git a/packages/main/src/request/index.ts b/packages/main/src/request/index.ts
index b50007d8..77d6116f 100644
--- a/packages/main/src/request/index.ts
+++ b/packages/main/src/request/index.ts
@@ -64,6 +64,20 @@ export const { actions, perform } = combineHandlers({
     })
   },
 
+  delFlags(
+    connection: Connection,
+    box: BoxSpecifier,
+    source: imap.MessageSource,
+    flags: string[]
+  ): R<void> {
+    return withBox(connection, box, () => {
+      const result = promises.lift0(cb =>
+        connection.delFlags(source, flags, cb)
+      )
+      return kefir.fromPromise(result)
+    })
+  },
+
   end(connection: Connection): R<undefined> {
     connection.end()
     return kefir.constant(undefined)
diff --git a/packages/main/src/request/testHelpers.ts b/packages/main/src/request/testHelpers.ts
index ebcc315f..bd68e908 100644
--- a/packages/main/src/request/testHelpers.ts
+++ b/packages/main/src/request/testHelpers.ts
@@ -29,6 +29,11 @@ export function mockConnection({
       cb(null as any)
     }
   )
+  mock(Connection.prototype.delFlags).mockImplementation(
+    (_source, _flags, cb) => {
+      cb(null as any)
+    }
+  )
   mock(Connection.prototype.getBoxes).mockImplementation((cb: any) => {
     cb(null, boxes)
   })
diff --git a/packages/main/src/resolvers/conversation.test.ts b/packages/main/src/resolvers/conversation.test.ts
index 182085f0..d2a7baaa 100644
--- a/packages/main/src/resolvers/conversation.test.ts
+++ b/packages/main/src/resolvers/conversation.test.ts
@@ -194,6 +194,35 @@ it("marks a conversation as read", async () => {
   })
 })
 
+it("marks a conversation as unread", async () => {
+  const result = await graphql(
+    schema,
+    `
+      mutation setIsRead($conversationId: ID!, $isRead: Boolean!) {
+        conversations {
+          setIsRead(id: $conversationId, isRead: $isRead) {
+            id
+            isRead
+          }
+        }
+      }
+    `,
+    null,
+    null,
+    { conversationId, isRead: false }
+  )
+  expect(result).toEqual({
+    data: {
+      conversations: {
+        setIsRead: {
+          id: conversationId,
+          isRead: false
+        }
+      }
+    }
+  })
+})
+
 afterEach(() => {
   db.prepare("delete from accounts").run()
 })
diff --git a/packages/main/src/resolvers/conversation.ts b/packages/main/src/resolvers/conversation.ts
index 072864ab..60c0d8c0 100644
--- a/packages/main/src/resolvers/conversation.ts
+++ b/packages/main/src/resolvers/conversation.ts
@@ -55,23 +55,21 @@ export const Conversation: ConversationResolvers = {
 
 export const ConversationMutations: ConversationMutationsResolvers = {
   async setIsRead(_parent, { id, isRead }) {
-    if (!isRead) {
-      throw new Error(
-        "Marking a conversation as unread is not yet implemented."
-      )
-    }
     const thread = getConversation(id)
     if (!thread) {
       throw new Error(`Cannot find conversation with ID, ${id}`)
     }
-    setIsRead(thread.messages)
+    setIsRead(thread.messages, isRead)
     return thread
   }
 }
 
-function setIsRead(messages: cache.Message[]) {
+function setIsRead(messages: cache.Message[], isRead: boolean) {
   const grouped = Seq(messages)
-    .filter(message => !cache.getFlags(message.id).includes("\\Seen"))
+    .filter(message => {
+      const seen = cache.getFlags(message.id).includes("\\Seen")
+      return isRead ? !seen : seen
+    })
     .groupBy(message => List([message.account_id, message.box_id] as const))
   for (const [grouping, msgs] of grouped) {
     const accountId = grouping.get(0)
@@ -81,7 +79,7 @@ function setIsRead(messages: cache.Message[]) {
       continue
     }
     const uids = msgs.map(message => message.uid).filter(nonNull)
-    if (!uids.isEmpty()) {
+    if (!uids.isEmpty() && isRead) {
       queue.enqueue(
         queue.actions.markAsRead({
           accountId: String(accountId),
@@ -89,6 +87,14 @@ function setIsRead(messages: cache.Message[]) {
           uids: uids.valueSeq().toArray()
         })
       )
+    } else if (!uids.isEmpty() && !isRead) {
+      queue.enqueue(
+        queue.actions.unmarkAsRead({
+          accountId: String(accountId),
+          box,
+          uids: uids.valueSeq().toArray()
+        })
+      )
     }
   }
 }