diff --git a/server/channels/app/export_test.go b/server/channels/app/export_test.go
index 5637be7dab8..241d166a838 100644
--- a/server/channels/app/export_test.go
+++ b/server/channels/app/export_test.go
@@ -1586,3 +1586,152 @@ func TestExportSchemes(t *testing.T) {
 		require.Equal(t, customTeamGuestRole.BuiltIn, importedTeamGuestRole.BuiltIn)
 	})
 }
+
+// TestExportDeactivatedUserDMs specifically tests the MM-43598 bug
+// by validating that direct messages from deactivated users are exported correctly
+func TestExportDeactivatedUserDMs(t *testing.T) {
+	th1 := Setup(t).InitBasic()
+	defer th1.TearDown()
+
+	// Create a DM Channel
+	user2 := th1.BasicUser2
+	dmChannel := th1.CreateDmChannel(user2)
+
+	// 1. First basic user (active) sends a message to user2 (who will later be deactivated)
+	initialMessage := "initial_message_from_basic_user"
+	initialPost := &model.Post{
+		ChannelId: dmChannel.Id,
+		Message:   initialMessage,
+		UserId:    th1.BasicUser.Id,
+	}
+	initialPostCreated, appErr := th1.App.CreatePost(th1.Context, initialPost, dmChannel, model.CreatePostFlags{SetOnline: true})
+	require.Nil(t, appErr)
+
+	// 2. Have user2 reply with TWO types of replies:
+
+	// 2a. User2 replies in a thread (to the initial message)
+	threadedReplyMessage := "threaded_reply_from_user2"
+	threadedReply := &model.Post{
+		ChannelId: dmChannel.Id,
+		Message:   threadedReplyMessage,
+		UserId:    user2.Id,
+		RootId:    initialPostCreated.Id, // This makes it a threaded reply
+	}
+	_, appErr = th1.App.CreatePost(th1.Context, threadedReply, dmChannel, model.CreatePostFlags{SetOnline: true})
+	require.Nil(t, appErr)
+
+	// 2b. User2 sends a standalone reply (NOT in a thread)
+	nonThreadedReplyMessage := "non_threaded_reply_from_user2"
+	nonThreadedReply := &model.Post{
+		ChannelId: dmChannel.Id,
+		Message:   nonThreadedReplyMessage,
+		UserId:    user2.Id,
+		// No RootId, making it a standalone message, not a thread reply
+	}
+	_, appErr = th1.App.CreatePost(th1.Context, nonThreadedReply, dmChannel, model.CreatePostFlags{SetOnline: true})
+	require.Nil(t, appErr)
+
+	// 3. Now deactivate user2
+	_, err := th1.App.UpdateActive(th1.Context, user2, false)
+	require.Nil(t, err)
+
+	// 4. Export data
+	var b bytes.Buffer
+	appErr = th1.App.BulkExport(th1.Context, &b, "somePath", nil, model.BulkExportOpts{})
+	require.Nil(t, appErr)
+
+	// 5. Make a copy of the buffer for export validation
+	var exportDataCopy bytes.Buffer
+	_, nErr := exportDataCopy.Write(b.Bytes())
+	require.NoError(t, nErr)
+
+	// 6. Validate export data directly to ensure both types of replies are present
+	scanner := bufio.NewScanner(&exportDataCopy)
+	foundThreadedReply := false
+	foundNonThreadedReply := false
+
+	for scanner.Scan() {
+		var line imports.LineImportData
+		err := json.Unmarshal(scanner.Bytes(), &line)
+		if err != nil {
+			continue
+		}
+
+		// Check for direct posts
+		if line.Type == "direct_post" && line.DirectPost != nil {
+			// Check for the non-threaded reply (the standalone message)
+			if line.DirectPost.Message != nil && *line.DirectPost.Message == nonThreadedReplyMessage {
+				foundNonThreadedReply = true
+				// Verify username is correctly preserved in export
+				require.Equal(t, user2.Username, *line.DirectPost.User,
+					"Deactivated user's username should be preserved in export for non-threaded reply")
+			}
+
+			// Check for the thread starter and its replies
+			if line.DirectPost.Message != nil && *line.DirectPost.Message == initialMessage {
+				// Check if the threaded reply is in the replies array
+				if line.DirectPost.Replies != nil {
+					for _, reply := range *line.DirectPost.Replies {
+						if reply.Message != nil && *reply.Message == threadedReplyMessage {
+							foundThreadedReply = true
+							require.Equal(t, user2.Username, *reply.User,
+								"Deactivated user's username should be preserved in export for threaded reply")
+						}
+					}
+				}
+			}
+		}
+	}
+
+	// This is key for testing MM-43598
+	require.True(t, foundNonThreadedReply,
+		"Non-threaded reply from deactivated user must be present in export data")
+	require.True(t, foundThreadedReply,
+		"Threaded reply from deactivated user must be present in export data")
+
+	// 7. Import data into a new instance
+	th2 := Setup(t)
+	defer th2.TearDown()
+
+	i, appErr := th2.App.BulkImport(th2.Context, &b, nil, false, 5)
+	require.Nil(t, appErr)
+	require.Equal(t, 0, i)
+
+	// 8. Verify the DM channel was imported
+	channels, nErr := th2.App.Srv().Store().Channel().GetAllDirectChannelsForExportAfter(1000, "00000000", false)
+	require.NoError(t, nErr)
+	require.Equal(t, 1, len(channels), "Direct channel should be imported")
+
+	// 9. Verify all posts were imported
+	posts, nErr := th2.App.Srv().Store().Post().GetPosts(model.GetPostsOptions{
+		ChannelId: channels[0].Id,
+		PerPage:   1000,
+	}, false, nil)
+	require.NoError(t, nErr)
+
+	// We should have exactly 3 posts
+	require.Equal(t, 3, len(posts.Posts), "Should have imported exactly 3 posts")
+
+	// 10. Specifically check that both types of replies are present
+	foundThreadedReplyInImport := false
+	foundNonThreadedReplyInImport := false
+
+	for _, post := range posts.Posts {
+		if post.Message == threadedReplyMessage {
+			foundThreadedReplyInImport = true
+			// Verify this is a reply in a thread
+			require.NotEmpty(t, post.RootId, "Threaded reply should have a RootId")
+		}
+		if post.Message == nonThreadedReplyMessage {
+			foundNonThreadedReplyInImport = true
+			// Verify this is NOT a reply in a thread
+			require.Empty(t, post.RootId, "Non-threaded reply should not have a RootId")
+		}
+	}
+
+	// This directly tests the issue in MM-43598
+	require.True(t, foundNonThreadedReplyInImport,
+		"Non-threaded reply from deactivated user should be imported")
+	require.True(t, foundThreadedReplyInImport,
+		"Threaded reply from deactivated user should be imported")
+}