I frequently setup Exchange servers for businesses so I am quite familiar with the necessary steps required to do it properly. One thing on an mail administrators setup check list is DNS. Without the proper DNS entries, no one will find your mail server and you won’t be able to send mail reliably. That being said, it is imperative that you have a proper reverse DNS setup in place. Without going to deep into the actual details, rDNS requires an A record pointing to your mail server’s IP, and a PTR record that resolves your mail server’s IP to the name of your A record. This ensures that any mail sent by your server can be resolved using rDNS to validate you as the source. Most servers reject mail from servers that do not have valid rDNS configurations, primarily as defense against spammers.