Consider voting this idea up.  Email deliverability is becoming more of an issue these days, so running your mail through a service where you control the IP is a key component.  If we can't authenticate, things get tough.

https://success.salesforce.com/ideaView?id=087300000006wu7

@* Salesforce Administrators * @Admin Group, Raleigh, US @Solo Admins @Salesforce People  @* Outlook/Teams, Gmail, and Inbox *