We were unable to perform a reverse lookup (PTR) on the IP Address of your mail server. This is a problem because many organizations will not accept email from a server without a PTR record.
We recommend performing a Reverse Lookup on the IP Address for your mail server to try to find the cause of the error. You will need to contact your ISP and ask them to setup a correct PTR record for you. You should ask that this record be the same as the hostname of your mail server. We also recommend configuring your server to include this name in your SMTP banner.
When a sending server makes a connection to the recipient server, the recipient server notes the sending IP address and performs a reverse lookup, called a PTR lookup, named after the type of DNS record used. If the result of the reverse lookup matches the result of a forward DNS Lookup, then it's much more likely that the message is legitimate. If the IP address doesn't match, it's much more likely that the sending address was spoofed and therefore much more likely that it's unwanted and could be considered spam.
An additional test that can be performed is an SMTP Banner check which compares the hostname announced by the server upon initial connection with the forward and reverse lookups.
Only the organization which controls an IP can configure PTR records. PTR record queries are sent to the owner of the IP address which is the ISP, unlike other DNS queries which are sent to the DNS server of whoever owns the domain.