Not receiving emails from a site form:
This issue was caused by the hostname of the server, which was dsw33.managed.domain.com, Update this to a vaild hostname like mail.thedomain.com and add the matching PTR (reverse DNS) record to match. You will need to add an A record at your DNS provider that points mail.thedomain.com to the IP if the server.
Check if the script is attempting to send with the From header as the user’s submitted e-mail address. This can cause issues as most major e-mail providers employ an e-mail feature known as DMARC to help reduce forging of their domains: