Hi Craig,
Thanks for the confirmation.
And according to the information of the issue now, the email services and mail flow should be configured correctly because emails can be sent from Outlook 2011. So it indicates that the issue is more related to the Outlook for Mac 15.4 client. And since no special deployment or configuration is used to set up the email account in the Outlook client, we have very limited resources to troubleshoot this issue from Office 365 forum side.
You can post this issue in the Outlook for Mac forum to get the dedicated assistance. Your understanding is highly appreciated.
Best Regards,
Henry Huang