Problem
In most cases, from Java based (like Portal) or ABAP based (ERP, CRM,...) systems, sending email is easy by just setting the SMTP server IP address.
But if you're using a hosted email service like us (we're using Google Apps), SMTP server doesn't allow to send emails directly. It requires you to set specific SMTP port, TLS and SSL as well as authentication.
So, for example, for sending emails from ERP systems, you just have SMTP Server address and Port area for settings via SCOT transaction.
Solution
To solve the problem, you can use IIS SMTP Server as an intermediate SMTP server between your SMTP server and local SAP System.
ERP, Portal >> -- send direct mail -- >> IIS 6/7 >> -- send with Authentication-- >> SMTP Server >> recipient.
source: sdn. sap. com