Some issue in sending email using php email function - php

Question

I am trying to send an email from a enquiry form available on a catalogue website on which I am working and found a strange issue.
On the same domain I have a file to test the email function with following code in it:
$sender = 'someone#somedomain.tld';
$recipient = 'name#gmail.com';
$subject = "php mail test";
$message = "php test message";
$headers = 'From:' . $sender;
if (mail($recipient, $subject, $message, $headers))
{
echo "Message accepted";
}
else
{
echo "Error: Message not accepted";
}
and its working fine. But the same code is not working on the index.php page where I have the enquiry form and also not showing any error at all (I have checked the error log. I have also tried by putting this code in a separate file and by including it on index.php but the same result).
<form name="frm_enquiry" id="frm_enquiry" method="post" autocomplete="off">
<div class="row">
<div class="col-lg-3 col-md-3">
<div class="form-group">
<label for="fullname">Name:</label>
<input type="text" class="form-control" id="fullname" name="fullname" placeholder="Enter your name">
</div>
</div>
<div class="col-lg-3 col-md-3">
<div class="form-group">
<label for="email">Email:</label>
<input type="email" class="form-control" id="email" name="email" placeholder="Enter your email">
</div>
</div>
<div class="col-lg-3 col-md-3">
<div class="form-group">
<label for="mobile">Mobile No:</label>
<input type="text" class="form-control" id="mobile" name="mobile" placeholder="Enter your mobile number">
</div>
</div>
<div class="col-lg-3 col-md-3">
<div class="form-group">
<label style="width: 100%;" for="mobile"> </label>
<button type="submit" class="btn btn-block btn-primary" name="submit">Submit</button>
</div>
</div>
</div>
</form>
php:
if( isset( $_REQUEST['submit'] ) )
{
$name = $_REQUEST['fullname'];
$email = $_REQUEST['email'];
$mobile = $_REQUEST['mobile'];
echo $name . ' : ' . $email . ' : ' . $mobile . '<br>';
$sender = 'someone#somedomain.tld';
$recipient = 'name#gmail.com';
$subject = "php mail test";
$message = "php test message";
$headers = 'From:' . $sender;
if (mail($recipient, $subject, $message, $headers))
{
echo "Message accepted";
}
}
Someone please guide me, what I am doing wrong here, I am stuck on this from last 2 days.
Thanks in advance.