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Method 1 – Opening the mailbox in its own window.
1. Open the Outlook Web Application via https://outlook.office.com/mail/
2. Click the thumbnail in the upper right-hand corner. This will either be your initials or a thumbnail image if you have set one. Select the “Open another mailbox” option.
3. Search for and select the mailbox you are attempting to open. Click Open.
The Mailbox will now open in its own tab.
Method 2 – Adding the mailbox as a folder within your own Outlook web client.
1. On the right-hand folder column, you’ll notice your own mailbox under the “Folders” section. Right-click on Folders and select Add shared folder or mailbox.
2. Search for and select the mailbox you are attempting to add. Click Add.
The mailbox will now be added to your right-column until you remove it manually.
3. When using this method, its important to know from which address you are sending email from. Click the gear icon in the upper right hand corner to access your settings. Navigate to Mail > Compose and reply > and check Always show From. Save.
4. You will now see the From drop down option. It's likely the mailbox you added is not in your list yet, if that is the case, just click Other email address...
5. Search for and select the mailbox you are sending from. (You will only need to do this once).
Now the email you compose will come from the mailbox you have selected (As long as you have been granted Send Permissions)
