It can take up to 24 hours for a shared mailbox to display in Outlook. But you can open the mailbox on Outlook web now using the below instructions:

1. login to outlook.office.com without your HPG email address

2. Click on your avatar (top right of the page)

3. Click Open another mailbox

4. Search for the email address of the mailbox you are trying to open

5. From the results, click on the Mailbox you want to open, and then click Open

The shared mailbox should open in a new tab.