How do I add and use an alternate or shared mailbox in Outlook (Desktop)?

After you have been added as member of another mailbox or shared mailbox, close and then restart Outlook. The shared mailbox should automatically display in your Folder pane in Outlook.

If you've only recently added you to a shared mailbox, it may take a few minutes before the shared mailbox appears. Wait and then close and restart Outlook again.

If that still didn't work, then manually add the shared mailbox to Outlook:

Open Outlook.
Choose the File tab in the ribbon.
Choose Account Settings, then select Account Settings from the menu.
Select the Email tab.
Make sure the correct account is highlighted, then choose Change.
Choose More Settings > Advanced > Add.
Type the email address of the account you'd like to add.
Choose OK > OK.
Choose Next > Finish (or Done) > Close.
You should now see the additional mailbox.

Sending mail from the shared mailbox:

Open Outlook.
Choose New Email.
If you don't see the From field at the top of your message, choose Options > From.
Click From in the message, and change to the shared email address. If you don't see your shared email address, choose Other email address and then type in the shared email address.
Choose OK.
Finish typing your message and then choose Send.
From now on, whenever you want to send a message from your shared mailbox, the address will be available in your From drop down list.
When you send a message from your shared mailbox, your recipients will only see the shared email address in the message.

Reply to mail sent to the shared mailbox

Open Outlook.
Open the email message you want to reply to and click Reply.
From field at the top of your message, you should see the name of your shared mailbox. If you don't, choose the From dropdown box and choose your shared mailbox.
Finish typing your message and then choose Send.
When you want to send a message from your shared mailbox, the address will be available in your From drop down list.
Whenever you send a message from your shared mailbox, your recipients will only see the shared email address in the message.

Use the shared calendar

In Outlook go to calendar view, and choose the shared mailbox.
Choose the calendar view
When you enter appointments, everyone who is a member of the shared mailbox will be able to see them.
Any member of the shared mailbox can create, view, and manage appointments on the calendar, just like they would their personal appointments. Everyone who is a member of shared mailbox can see their changes to the shared calendar.

Use the shared contact list

After you've been added you as a member of shared mailbox, close and then restart Outlook. The shared contact list associated with the shared mailbox is automatically added to your My Contacts list.
In Outlook, choose People.
Under My Contacts, choose the contacts folder for the shared contacts list.
Shared contacts list displays in Contacts Pane in Outlook