In our hosted environment, we utilize a robust email system that will deliver email to any recipient.  However, while the email is being delivered with a "Send From" address with your own district email domain, the message is physically coming from us.  This requires the district to create a SPF record (or append an existing record) which will include the following host name _spf.aeries.net.

In order for an SPF record to be effective, it is important to use a real domain name* in your "Send Emails From" setting in the Portal Settings section of the School Options page.  If you are using Online Enrollment (AIR), you will also need to verify the same setting there, too.