why droplet in digital Ocean can't send php mail to hotmail and gmail ?

why droplet in digital Ocean can’t send php mail to hotmail and gmail ? Hotmail and gmail block mail form digital Ocean ??
my digitalOcean http://128.199.146.94/omnama.com/

I’m beginner use ubuntu on my website but I don’t know how to set ubuntu sent mail to gmail ,hotmail?

I’m install sudo apt-get install postfix set by internet after I’m use this code but Why log tell me have sent to sawasdee@outlook.com But I check in hotmail and gmail can’t see my email. why php mail can’t sent ???

this log and coding:

<?php 
$strTo ="aroidee@live.com";
    $strSubject = "=?UTF-8?B?".base64_encode("mail1 hello")."?=";
    $strHeader .= "MIME-Version: 1.0' . \r\n";
    $strHeader .= "Content-type: text/html; charset=utf-8\r\n"; 
    $strHeader .= "From: sawasdee company<sawasdee@outlook.com>\r\nReply-To: sawasdee@outlook.com";
    $strMessage = "
      <body>
    hello world!
 </body>";

    $flgSend = @mail($strTo,$strSubject,$strMessage,$strHeader);  // @ = No Show Error //
    unset($flgSend,$strTo,$strSubject,$strHeader,$strMessage);


    $strTo ="sawasdee@outlook.com";
    $strSubject = "=?UTF-8?B?".base64_encode("mail2 sawasdee")."?=";
    $strHeader .= "MIME-Version: 1.0' . \r\n";
    $strHeader .= "Content-type: text/html; charset=utf-8\r\n"; 
    $strHeader .= "From: sawasdee company<sawasdee@outlook.com>\r\nReply-To: sawasdee@outlook.com";
    $strMessage = "
      <body>
      sawasdee world!</body>";
        $flgSend = @mail($strTo,$strSubject,$strMessage,$strHeader);  // @ = No Show Error //
    unset($flgSend,$strTo,$strSubject,$strHeader,$strMessage);

?>

Why log show can sent complete but I check in hotmail and gmail can’t see in junk ,Trash,inbox not found?

Jul 30 05:47:17 sawasdee1 postfix/pickup[24607]: D79DF120472: uid=33 from=<www-data>
Jul 30 05:47:17 sawasdee1 postfix/cleanup[24669]: D79DF120472: message-id=<20150730094717.D79DF120472@sawasdee1>
Jul 30 05:47:17 sawasdee1 postfix/qmgr[24608]: D79DF120472: from=<www-data@sawasdee1>, size=500, nrcpt=1 (queue active)
Jul 30 05:47:17 sawasdee1 postfix/pickup[24607]: DA29812006F: uid=33 from=<www-data>
Jul 30 05:47:17 sawasdee1 postfix/cleanup[24669]: DA29812006F: message-id=<20150730094717.DA29812006F@sawasdee1>
Jul 30 05:47:17 sawasdee1 postfix/qmgr[24608]: DA29812006F: from=<www-data@sawasdee1>, size=505, nrcpt=1 (queue active)
Jul 30 05:47:19 sawasdee1 postfix/smtp[24675]: DA29812006F: to=<aroidee@live.com>, relay=mx1.hotmail.com[65.54.188.110]:25, delay=1.2, delays=0/0.01/0.58/0.62, dsn=2.0.0, status=sent (250  <20150730094717.DA29812006F@sawasdee1> Queued mail for delivery)
Jul 30 05:47:19 sawasdee1 postfix/qmgr[24608]: DA29812006F: removed
Jul 30 05:47:19 sawasdee1 postfix/smtp[24674]: D79DF120472: to=<sawasdee@outlook.com>, relay=mx1.hotmail.com[65.55.92.168]:25, delay=1.5, delays=0.02/0.01/0.65/0.79, dsn=2.0.0, status=sent (250  <20150730094717.D79DF120472@sawasdee1> Queued mail for delivery)
Jul 30 05:47:19 sawasdee1 postfix/qmgr[24608]: D79DF120472: removed