To move the Outlook toolbar (navigation bar with Mail, Calendar, etc.) from the side back to the bottom, follow these steps in the classic Outlook desktop app:

1. Open Outlook.
2. Click File in the top-left corner.
3. Select Options from the menu.
4. In the Outlook Options window, click Advanced in the left sidebar.
5. Under the Outlook panes section, uncheck the box labeled Show Apps in Outlook.
6. Click OK to save your changes.
7. Restart Outlook when prompted.

After restarting, the toolbar will move from the left sidebar to the bottom of the Outlook window, restoring the classic layout many users prefer

. Note:

• This method works only in the classic (old) Outlook interface. In the new Outlook interface, the toolbar is fixed on the left side and cannot be moved.
• If you want to revert the toolbar back to the left side, simply check the Show Apps in Outlook option again and restart Outlook

Alternatively, if the option is not available or does not work, a registry tweak can be applied, but it is recommended only for advanced users due to risks involved with editing the Windows registry