SMTP settings are used to send a Notification alert via Email of various events to your end-user in iTextPro. Admin needs to make sure SMTP settings are correctly configured and working to receive system alerts.
You can visit www.gmass.co/smtp-test or www.smtper.net etc to simulate the SMTP settings details, once you are able to recieve the test emails, then you can configure it in site branding option of iTextPro application.