Updating cached headers outlook 2016
Unfortunately, by default Outlook cached mode also includes the Global Address List.This can mean that new accounts or distribution lists are not available to all clients straight away.It will not work with accounts as they do not support online mode.By default, Outlook sets Exchange accounts up in cached mode.
The Outlook cached mode feature was designed to reduce network traffic between the Outlook clients and the Exchange server.
You can delay sending mail for a few seconds (to prevent accidental sends) but cannot configure a message to send later, as you can in Outlook on Windows.
Because you are using a Microsoft Exchange mailbox (either on-prem or Office 365 Exchange online) you can send messages later, with Outlook closed, as long as you can set up the account in Outlook desktop using Online mode.
You can check in File, Account Settings: open the account settings dialog, and double click on the account.
Cached will be ticked by default when you set up the account, untick it to drop to online mode.