PHP Site - Check Email Sending Code

I have a PHP jobs site which sends out emails: 1. To users (jobseekers) when they register and confirmation of application for jobs. 2. To admins (owners of the website with email addresses based on domain) from jobseekers of the applications made. Now I have checked the automatic email sending code and tested the system: 1. As a jobseeker 2. As an admin but with a EMAIL ADDRESS NOT BASED ON THE DOMAIN In all the cases emails were being received from the website. Now when I add an admin with an email address based on the DOMAIN of the website, the admin emails are not being received. I think this problem is because the website is not programmed to send via SMTP. I need someone to change the email sending function to SMTP and test it. Pretty quick job as I think I have identified the problem. Need this done urgently.