We have an in-house Ubuntu server (hosting a Drupal-based intranet site) and in-house Exchange 2013 server. The Ubuntu server has postfix installed and can successfully email external addresses (e.g. my gmail account). Messages directed to the in-house Exchange server however seem to black hole (no record in the message logs).
I have a custom receive connector that allows for basic authentication from specified IP addresses within the domain (Basic authentication only; allowed from all permission groups). Other servers in the list are able to send mail successfully.
Any recommendations on what I might be missing here? Thank you very much in advance.