There are two ways you can open a shared mailbox using Outlook Web Access (OWA):
This method will ensure that the default From address and Sent and Deleted Items folders that are used are the ones belonging to the shared mailbox.
When using this method, sent messages or deleted messages will go into the folders in your individual mailbox. Also, you will need to ensure that you change the From address when composing a message.
The shared mailbox address will need to be manually added to the “From” options the first time you send from it. After that, it will regularly appear as one of the “From” options.