Thank you for information. Our situation is that it's the web server that is trusted. Content management system on the web server generates notification e-mails (using PHP's mail() function). Those e-mails are then sent via sendmail but I can't say for sure if sendmail acts as a relay or as an outgoing host. That's why we've set the server as trusted.

 

Maybe you know if it's a relay or not and can give us a hint on whether we need to keep this option.