View Delegated or Shared Mailbox

A shared mailbox makes it easy for a group of people to monitor and send email from a public email alias like info@contoso.com. When a person in the group replies to a message sent to the shared mailbox, the email appears to be from the shared address, not from the individual user.

Delegated Mailbox can be used in similar fashion, but it does belong to a specific user, and they have more control over their own email account.

Options to access another mailbox

*All these options do require admin or the owner of the mailbox assigning you access

Online Outlook

https://outlook.office.com/mail/

Quickest and easiest method would be to login to your email using link above, clicking on your initials in top right corner and clicking to "Open another mailbox"

Then type in and choose the email address and press open

More permanent option - (New Outlook and Outlook Web)

After your admin has added you as a member of a shared mailbox, close and then reopen Outlook. The shared mailbox should automatically display in your Folder pane.

What if it didn't work?

Add the shared box manually.

Select Mail from the navigation pane in new Outlook.
In the Folder pane, right-click your account name, and select Add shared folder or mailbox.
In the Add shared folder or mailbox window, type the name of the mailbox you want to access, for example, info@contoso.com.

Outlook Classic Desktop

With regular Desktop Outlook installed, when you are delegated access to inbox, it may take an hour or so after its applied, it should automatically appear on the sidebar as another set of email folders. Sometimes you may need to fully close out of outlook and reopen it, and generally you do need to click the > icon to drop down the folders.

For More Information: Open and use a shared mailbox in Outlook - Microsoft Support

Use a shared mailbox on a mobile device (phone or tablet)

To access a shared mailbox on a mobile device, use the following steps.

Sign in to your primary account in Outlook for iOS or Android.
Tap the Add Account button in the left navigation pane, then tap Add a Shared Mailbox.
If you have multiple accounts in Outlook Mobile, select the account that has permissions to access the shared mailbox.

After the account setup process completes, the shared mailbox will display in your account list in Outlook for iOS or Outlook for Android.

To remove a shared mailbox, go to Settings > Accounts. Then tap on the shared mailbox and select Delete Account.