Email delivery from third party servers sending on behalf of a user's email address requires a few things to be done in order for delivery to be reliable. Mail servers are using more strict controls to combat SPAM and other abusive email content. Here's what you need to do so that your emails will have the highest rate of delivery.
Update (AUG 25 2018):
The following tutorial will show you the best way to configure your email settings using either Domain Validation or sending through your own SMTP service.