1. Open a new email

2. Delete the email for the shared mailbox for the "From" dropdown menu using the X 

2. Exit out of the new email window

3. Re-enter the new email window

4. Select from and then "Other email address". Then click "From" again and search for the mailbox and then click ok and ok again.

5. Go to File, Account Settings and then Account Settings and then double click your email

6. Untick "Use cached exchange mode"

7. Click Next and then Done and then restart Outlook.