Normally email sent from your accepted domains (as defined in Exchange Server) will only be generated from within your own network, however you may want to allow external servers (e.g. web servers) to send mail to your users with an accepted domain address, but this inbound mail will be subject to spam filtering and may end up in the user’s Junk Email folder in Outlook (or worse!).
To allow the external server generated mail to bypass the filtering, simply go in to EMC (Exchange Management Console) and enable the IP allow list in Organisation Configuration, Hub Transport, Antispam.
Then add the server(s) IP address(es) to the list under Server Configuration, Hub Transport, Antispam.