Hi guys,
I want to configure the relay for my Zimbra server
Only allow and

I guess the setup will be place here,
Server settings > MTA > MTA Trusted Networks >

But right now I run a test and facing a problem that I configure the SMTP sending mail by using the PHP code, the code similar to below,

$mail             = new PHPMailer();

	$mail->Host       = "mail.abc.com"; 
	$mail->CharSet = 'utf-8'; // SET UTF-8
	$mail->From       = "andy@abc.com";
	$mail->FromName   = "ABC Pte Limited";

	$mail->Subject    = "[Testing]";

	$mail->AddAddress($emailadd, "");
The server IP is, and send notification using andy@abc.com and sending email to other mail provider, example email notification.
and it is send successful.

Here is the question,
I do not add as my relay and why it is able to send? can I disallow the server using our email server to send email out?

Hope someone expert can give me advice on this question.