When adding a user with "Send As" permissions to a mailbox in Exchange, this can cause the Outlook Automatic Pushout to break and not add the mailbox.

To fix this we use ADSI edit to remove the new user from the pushout list.

• Open ADSI Edit on the exchange server.
• Navigate to the relevant OU of the shared mailbox, and selected the CN for example CN=TestSharedMailbox
• Right click and select "Properties" to bring up the Attribute Editor.
• Find msExchDelegateListLink attribute
• Select Edit and remove any values in there
• Click ok

Now they've been removed from the auto push-out. You will need to manually add the mailbox in Outlook. Either through

Control Panel - Mail\Email Accounts\New\ then enter the shared address with no password

or 

 Outlook - File\Account Settings\Account Settings\Profile e.g ShayN@abc-data.co.uk\ More Settings\Advanced\Add these Mailboxes