Send Two Emails In A Same Time

I just wanna know what's the php code to send two emails in a same time

I have this code:
$email1 = "....etc....";
$headers1 = ".... etc...";
$subject1 = "....etc...";
$message1 = "....etc...";

$email2 = "....etc....";
$headers2 = ".... etc...";
$subject2 = "....etc...";
$message2 = "....etc...";

mail($email1, $subject1, $message1, $headers1) or die('Failed');
mail($email2, $subject2, $message2, $headers2) or die('Failed');

but it doesn't work, why?

What's the error message or what's going wrong precisely ?

It always Failed

Message not send if I use to mail() in a same time. If I use only one mail() the message was sent successfully

May be the SMTP server has some limitations ? (to avoid spam)

Have you try to put some delay between the two mail() command ?

Something like

sleep(5);

to pause for 5 seconds between the two mail() calls ?