Why Headers Preventing mail from sending?

Question

This might sound similar like previously asked questions but trust me it's not

I Was trying to send an email that uses an HTML template via PHP mail() function from Localhost and a Hostinger Server but they created different problems.

1. On localhost the email was being sent as plain text although there were headers

    $headers =
        "MIME-Version: 1.0\r\n" . 
        "Content-Type: text/html; charset=UTF-8";
   

   I have gone through all the similar questions in stackoverflow and tried each and every thing but I couldn't make it work. After some more research on this I found out this

   I assume, that your email client is considering the smtp-server "unsafe", and hence is just going to display all the html as plaintext, rather than rendering it

2. Therefore I switched over my hosting and tried to do the same but this time I found that the headers are causing the problem. The email is not sent if the header variable is passed in the mail() function. I tried to concatenate the headers which didn't worked. Then I made an array of headers and joined them with php implode which too didn't worked. On a similar question on stackoverflow I found that webmails mess up if html, head, body tags are used as they use xhtml. I removed them and still no success.

I tried error reporting too and it showed module sqlite3 already loaded which I think is not related to mail.

Below is my code

php

<?php
 $email_template = file_get_contents("path/to/my/template");
 $lucky_number = rand(999999, 111111);
 $email_template = str_replace("{{user}}", "User", $email_template);
 $email_template = str_replace("{{lucky_number}}", $lucky_number, $email_template);
 $sender = "from:iusername@host.com"; // I found that if I dont use from, my mail ends up in spam folder
 $receiver = "username@host.com";
 $subject = "Random Subject Name";
 $headers =
    "MIME-Version: 1.0\r\n" . 
    "Content-Type: text/html; charset=UTF-8";

 if(mail($receiver, $subject, $email_template, $sender, $headers))
 {
    echo "Email Sent Successfully";
 }
 else
 {
    echo "Email Sending Failed";
 }

P.S I can't use PHPMailer or other similar libraries

Answer 1

The sender information should be inside the headers

Hence, please change the following lines:

 $headers =
    "MIME-Version: 1.0\r\n" . 
    "Content-Type: text/html; charset=UTF-8";

 if(mail($receiver, $subject, $email_template, $sender, $headers))
 

to

$sender = "iusername@host.com";

$headers = "From: $sender <$sender>\r\nReply-To: $sender\r\n";
$headers .= "MIME-Version: 1.0\r\n";
$headers .= "Content-Type: text/html; charset=utf-8\r\n";

 if(mail($receiver, $subject, $email_template, $headers))