The documentation includes the chapter Configuring SMTP variables (optional) to configure the SMTP server used to send invitations. However there is an extra step missing which is editing the field Outgoing Email from: 

Admin Portal > Audience > Developer Portal > Settings > Domains & Access

or from the browser to https://<ADMIN_PORTAL_URL>/site/dns

Additionally other environment variables should be set to configure correctly the: 

• Notification emails
• Support emails
• Invitation emails from Master portal

as described in this KCS:
Editing the outgoing email address for the 3scale Admin/Master Portals.

The request is to include the information from the KCS in our documentation.