Why php mail has delay when sending message with images - html

Question

I was building a email functionality for my website and I am using PHP mail function. The issue I am having is that when I try to email a client with a image inside a message it takes forever(20-45 minutes) to get to them and when I just include text it gets to them right away. Is there solution to this. Thanks for any help.
<?php
$email = $_COOKIE["email"];
$link = $_COOKIE["coupon"];
$to = $email;
$subject = 'Your ads';
$message = ' Hello This is Testing Email 3.0 Text & Image Your Coupon Link
<img src="$link" width="300" height="300"/> ';
$headers = 'MIME-Version: 1.0' . "\r\n";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
$headers .= 'From: Ads <ads#advertising.com>' . "\r\n";
mail($email, $subject, $message, $headers);
?>

Answer 1

According to your code, you mail is a top tier Spam grade for almost all anti spamming mailing protection.
They will most likely put your mail in a slow queue because of that, delaying the message because it was considered an annoyance.
Your From header contains 'ads' and 'advertising' (even if I guess that advertising.com isn't your domain.
You also have little to no text, the word test in it and a big link button named "coupon".
You should try to make your email more personal.
This is the most likely problem.
Second one would be the file transfer.
If you're transferring from China to New-York with a 56kb/s connection, the file transfer will take long enough for your recipient to die from old age.
For your second problem, replace
$message = ' Hello This is Testing Email 3.0 Text & Image Your Coupon Link
<img src="$link" width="300" height="300"/> ';
by
$message = ' Hello This is Testing Email 3.0 Text & Image Your Coupon Link
<img src="' . $link . '" width="300" height="300"/> ';