Pressing the actionbar icon will take you back to the accountsscreen.

src/com/fsck/k9/activity/Accounts.java

@@ -309,6 +309,13 @@ public class Accounts extends K9ListActivity implements OnItemClickListener, OnC
         context.startActivity(intent);
     }
 
+    public static void listAccountsClearTop(Context context) {
+        Intent intent = new Intent(context, Accounts.class);
+        intent.putExtra(EXTRA_STARTUP, false);
+        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
+        context.startActivity(intent);
+    }
+
     @Override
     public void onNewIntent(Intent intent) {
         Uri uri = intent.getData();

src/com/fsck/k9/activity/FolderList.java

@@ -556,6 +556,12 @@ public class FolderList extends K9ListActivity implements OnNavigationListener {
 
     @Override public boolean onOptionsItemSelected(MenuItem item) {
         switch (item.getItemId()) {
+        case android.R.id.home:
+            Accounts.listAccountsClearTop(this);
+            finish();
+
+            return true;
+
         case R.id.compose:
             MessageCompose.actionCompose(this, mAccount);
 

src/com/fsck/k9/activity/MessageList.java

@@ -1500,6 +1500,12 @@ public class MessageList
         final List<MessageInfoHolder> selection = getSelectionFromCheckboxes();
         int itemId = item.getItemId();
         switch (itemId) {
+        case android.R.id.home: {
+            // app icon in action bar clicked; go home
+            Accounts.listAccountsClearTop(this);
+            finish();
+            return true;
+        }
         case R.id.compose: {
             onCompose();
             return true;

src/com/fsck/k9/activity/MessageView.java

@@ -852,6 +852,10 @@ public class MessageView extends K9Activity implements OnClickListener {
     @Override
     public boolean onOptionsItemSelected(MenuItem item) {
         switch (item.getItemId()) {
+        case android.R.id.home:
+            Accounts.listAccountsClearTop(this);
+            finish();
+            break;
         case R.id.delete:
             onDelete();
             break;