You can select all items in an Outlook folder quickly with shortcut keyboards. And there are three sets of shortcut keyboards for you:
    A. Ctrl + A: Open the specified folder in Outlook, select any email (or contact, calendar items, tasks, etc.) in it, and then press Ctrl + A keys simultaneously.
    B. Ctrl + Shift + End: Open the specified folder in Outlook, select the first item in it, and then press Ctrl + Shift + End keys simultaneously.
    C. Shift: Open the specified folder in Outlook, select the first item, and then select the last item with holding the Shift key.

Quickly search and delete duplicate emails in Outlook

With Kutools for Outlook’s Duplicate Emails feature, you can quickly find and delete them from multiple mail folders, or find and delete all duplicates from the selected ones with two clicks in Outlook.

For selecting all emails, contacts or calendar items in Outlook 2010 or higher versions, please do as follows.

1. Click the arrow on the Quick Access Toolbar at the top of Outlook interface, and then select More Commands from the drop down list. See screenshot:

2. In the Outlook Options dialog box, you need to (1) select All Commands in the Choose commands from drop-down list, (2) scroll down to choose Select All in below list box, (3) click the Add button, and finally (4) click the OK button. See screenshot:

3. Then the Select All button is added on the Quick Access Toolbar. See screenshot:

Notes:
    (1). When you want to select all items such as all emails in a specified folder, please get into this folder and then click the Select All button.
    (2). After adding the Select All button, it can be applied in all Outlook folders.

If you are using Outlook 2007, you can add the Select All button in the Quick Access Toolbar with below steps:

1. Firstly you need to add the Advanced Toolbar in the Outlook Ribbon. Please right click on the Outlook ribbon and then check the Advanced in the right-clicking menu.

Then you can see the Advanced toolbar is added to the Outlook ribbon. See right screenshot:

2. Get into the Outlook folder which you want to select all items from. Right click the Advanced toolbar, and then click Customize from the context menu.

3. In the Customize dialog box, you need to (1) click the Commands tab, (2) click Edit in the Categories box, and then (3) drag the Select All out of the Commands box to the advanced toolbar. See screenshot:

Then the Select All button is added in the toolbar.

Note: In Outlook 2007, if you are locating in the Mail and Contact view, the Select All button is displayed normally. If you want to select all calendar items, you need to change the calendar view to list view to show the Select All button.