You might be aware that using File > Open > Other User to open a shared mailbox will only show you the mailbox’s Inbox folder.
Here’s how to have full access using an Exchange Online connection (it works with IMAP/POP/SMTP if you have those enabled as well, but most will prefer Exchange):
- Go to Preferences > Accounts
- Add a new account
- Enter you shared mailbox full email
- Manually choose Office565 as the email provider
- You will now be presented with the Office365 login popup. Under the password field choose Use another account
- Enter you own username (Office 365 email) and then your password.
- And that’s it! You should have your Shared Mailbox fully available