title: PHP mail function not sending emails to certain addresses
body: <p>I've been using the php mail function to send emails containing account details to clients when they are registered but I have only just found out that some people are not receiving emails. E.g. when I send an email to each of my collegues when testing only me and one other collegue are receiving the email </p>
  
  <pre><code>- me who received email
  - pam@companyx.co.uk
  - collegue who received email
  - bill@companyx.co.uk
  - collegue who did not receive emails:
  - email 1 - ted@companyx.co.uk,
  - email 2 - ted@companyy.co.uk
  - collegue who did not receive emails:
  - cal@companyy.co.uk
  - collegue who did not receive emails:
  - emma@companyy.co.uk
  </code></pre>
  
  <p>The emails are not received in the spam folder either..</p>
  
  <pre><code>$from = "companyy &lt;support@companyy.co.uk&gt;";          
  $subject = "$username Account Details for companyy.co.uk";     
  $message = "You have been registered as an administrator of companyy Online.
  You can manage your clients, bills and other account functions online.       
  
  Your Username: $username
  Your Password: $password2
  Your Four-Digit PIN: $PIN         
  
  Once logged in you can change your password to something you will find easier to remember. Please delete this email once you have changed your settings or memorised them.           
  
  Thank you        
  
  This is an automated response, please do not reply!";
  
  mail($email, $subject, $message, $from);
  Header('Location: results.php?msg=Added Successfully');
  </code></pre>