Emails to the sent items folder of a Shared Mailbox

When an user comes to you and tells you, his email that is sent from a Shared Mailbox comes in the their personal sent items folder.

msExchHideFromAddressLists don’t sync to Exchange Online

You run into an issue when you want to remove a synced recipient from the GAL even if you set msExchHideFromAddressLists on $true