php_send_mail_function problem

Hi all, The php send mail function on our CMS has been working perfectly fine until someone changed the MX record on the domain control panel. Since then emails are no longer receiving although it seems to send without prompting any error message. The qmail service runs perfectly fine on the server. Since we didn't change anything on PHP end, we don't think it is PHP issue. It is more of a server issue (Linux). We use Virtuozzo/ Plesk 8.0.1. Please get in touch if you have experienced this sort of problem before and know how to fix this issue. Regards, Tin