PHPMailer sending e-mails with the warning: This message may not have been sent by: foo@gmail.com Learn more Report phishing

I am using PHPMailer to send automated e-mails from my website and while testing, I noticed that all e-mails sent by php mailer are generating the following warning on the recipients end:

This message may not have been sent by: foo@gmail.com Learn more Report phishing

I was wondering if there is a way to avoid this?

PHP Mailer code:

//mail functions
require("mailer/class.phpmailer.php");
require("mailer/class.smtp.php");
require("mailer/class.pop3.php");

$mail = new PHPMailer();
$mail->IsSMTP();  
$mail->Host = "relay-hosting.secureserver.net";
$mail->Port = 25;  
$mail->IsHTML(true);
$mail->Username = "foo@gmail.com";  // SMTP username
$mail->Password = "pass"; // SMTP password

$mail->From = "foo@gmail.com";
$mail->FromName = "FOO";
$mail->AddAddress("fOO@gmail.com", "WIDB");
$mail->AddReplyTo("foo@gmail.com");
//$mail->AddAddress("foo@gmail.com");                  // name is optional

$mail->WordWrap = 50;                                 // set word wrap to 50 characters
//$mail->AddAttachment("/var/tmp/file.tar.gz");         // add attachments
//$mail->AddAttachment("/tmp/image.jpg", "new.jpg");    // optional name
$mail->IsHTML(true);                                  // set email format to HTML

$mail->Subject = 'Foo - Transaction Receipt';
$mail->Body    = $message;
$mail->AltBody = "nothing";

//send mail
$mail->Send();

I am using GMail and I have SMTP enabled...