$ch = curl_init();
curl_setopt($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);
以上是我的发送功能的片段。如果我在 otherdomain 上使用 gmail、yahoo 或 aol,则发送消息。但如果我使用自定义 otherdomain,它会返回一个 Server Response: 550
我错过了什么?