$from = "Info <donotreply@test.com>";
$subject = "Calibration will be expiring!";

$body = 'Hello';

$to = "david@test.com, dono@test.com";
$cc = "rena@test.com";
$bcc = ""; //sometimes BCC can be empty

$host = 'smtp.test.com';
$port = '587';
$username = 'donotreply@test.com';
$password = 'dontreply?';

$headers = array(
 'Port'          => $port,
 'From'          => $from,
 'To'            => $to,
 'Subject'       => $subject,
 'Content-Type'  => 'text/html; charset=UTF-8',
 'Cc'            => $cc
);

$recipients = $to.", ".$bcc.", ".$cc;

$smtp = Mail::factory('smtp',
    array ('host' => $host,
    'auth' => true,
    'username' => $username,
    'password' => $password));

$mail = $smtp->send($recipients, $headers, $body);

if (PEAR::isError($mail)) {
    echo("<p>" . $mail->getMessage() . "</p>");
} else {
 echo("<p>Message successfully sent!</p>");
}

Failed to add recipient: @localhost [SMTP: Invalid response code received from server (code: 501, response: 5.1.3 Invalid address [HKXPR02MB0695.apcprd02.prod.outlook.com])]