Cannot use variable $_SERVER['HTTP_HOST'] in php mail() - php

Question

I tested this:
<?php
$to = "recipient#example.com";
$subject = "Hi!";
$server = $_SERVER['HTTP_HOST'];
$body = "From: ". $server. "<br>";
$body .= "Hi,\n\nHow are you?";
if (mail($to, $subject, $body)) {
echo("<p>Message successfully sent!</p>");
} else {
echo("<p>Message delivery failed...</p>");
}
?>
This will never send emails to me although the feedback "successfully" is displayed.
The code will work (email actually sent) however if I removed the inclusion of
$server = $_SERVER['HTTP_HOST'];
in the email body.
Very weird, it doesn't make sense ?
This is just a PHP page. I call this page from a browser !! Please try ...
UPDATE!! okay, Instead of using $_SERVER['HTTP_HOST'], I use a string "user.server.com" directly. AND, it did not work !!
But, when I modified a string a bit, like "user.server.com.us", it works !!
So basically, the mail server filler its own reference to its domain, not sure why its doing this...

Answer 1

Some spam servers grade your message as 'spammy' for urls in the body. Unfortunately you'll be hard-pressed to find a more specific error message with mail() unless you have access to the sender and recipient's mail server logs. Give SwiftMailer or another PHP Mailing library for better support.

Answer 2

When using mail() function you are required to specify 'From' header. Please refer to this documentation page, look for 'additional_headers' parameter's Notes section.
So your mail() call will look like this:
mail($to, $subject, $body, 'From: some.guy#example.com');

Answer 3

Of course you don't receive it because there these days all major webmails use anti spam filters and probably your message is detected as a spam.
The solution is to specify exactly your email for $from variable, if you don't specify your real email your message is detected as spam.