php - Mail Gun Sending via HTTP returns Server response: 550 5.1.0 Recipient rejected -

$ch = curl_init();          curl_setopt($ch, curlopt_httpauth, curlauth_basic);         curl_setopt($ch, curlopt_userpwd, 'api:key-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx');         curl_setopt($ch, curlopt_returntransfer, 1);          curl_setopt($ch, curlopt_customrequest, 'post');         curl_setopt($ch, curlopt_url, 'https://api.mailgun.net/v2/domain.com/messages');         curl_setopt($ch, curlopt_postfields, array('from' => 'excited user <administrator@domain.com>',                                                  'to' => 'tester@otherdomain.com',                                                  'subject' => 'test',                                                  'text' => 'message'));          $result = curl_exec($ch); 

above snippet of send function. if user gmail, or yahoo or aol on otherdomain, message sent. if use custom otherdomain, returns server response: 550

what did miss?

i found out when use mailgun's incomming smtp, previois emails not available. have input them again on mailbox.