php - email sends to .com emails not .co.uk -

i have tried use simple contact form registration form.

when recipient email .com receive email after 30 seconds 10 minutes later fine after if email recipient .co.uk email never receive email have tried on 3 emails .co.uk ending , have sent 10 emails in total .co.uk email addresses.

i tested yesterday left 24 hours make sure wasn't coming through late have checked junk , spam mail boxes , there nothing blocking example firewall or anti-virus ideas appreciated code below , yes aware of myssql injection , other techniques or security not issue , sorted later on.

<h2>feedback form</h2> <?php // display form if user has not clicked submit if (!isset($_post["submit"]))   {   ?>   <form method="post" action="<?php echo $_server["php_self"];?>">   subject: <input type="text" name="subject"><br>   message: <textarea rows="10" cols="40" name="message"></textarea><br>   <input type="submit" name="submit" value="submit feedback">   </form>   <?php    } else   // user has submitted form   {   // check if "subject" input field filled out   if (isset($_post["subject"]))     {     $subject = $_post["subject"];     $message = $_post["message"];     // message lines should not exceed 70 characters (php rule), wrap     $message = wordwrap($message, 70);     // send mail     mail("webmaster@example.co.uk",$subject,$message,"subject: $subject\n");     echo "thank sending feedback";     }   } ?>