You should first check with your hosting company to ensure that the PHP sendmail function is enabled and working. They can check from their side very easily. You should also ask them if there has been a policy change which may restrict the forwarding of emails to addresses outside of the domain (on the assumption you may be using an email address that isn’t associated with your domain - i.e. gmail or some other) If all is ok at the hosting end, you should check that your site is fully uploading and isn’t leaving anything behind. If all else fails, it may be best to use the SMTP mail function so that replies get routed through one of your standard email accounts.