When creating a shared mailbox in Office 365 the default behaviour for items sent from this mailbox is odd. When a user who has access to the shared mailbox sends an item from or on behalf of the mailbox the item goes into the users ‘sent items’ folder. The sent item will not appear in the ‘sent items’ folder of the shared mailbox.

There is no obvious way to configure items sent from a shared mailbox to appear in the ‘sent items’ folder of the shared mailbox. This however can be configured quite easily from PowerShell. To do this you will first need to establish an Office 365 session in PowerShell. To establish a session:

Get Credentials

$cred = get-credential

Create a session with ps.outlook.com using the saved credentials

$session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell -Credential $cred -Authentication Basic -AllowRedirection 

Import the session

Import-PSSession $session -AllowClobber 

Once you have established the connection to Office 365 you can configure the ‘sent items’ behaviour or shared mailboxes.
To configure emails that are Sent As:

set-mailbox <mailbox name> -MessageCopyForSentAsEnabled $True

To configure emails that are sent of behalf of:

set-mailbox <mailbox name> -MessageCopyForSendOnBehalfEnabled $True

If you later want to stop the shared mailbox’s from saving sent items into their own ‘sent items’ folder, you can simple set the values in the commands above to $false instead of $true.