Trend Micro Email Security allows you to add reverse DNS validation for sender domains.
Two rules are pre-configured:
Default (for unspecified domains): applies to all sender domains, except those for which you have configured a new reverse DNS validation rule.
Empty sender: applies to email messages with no envelope address specified.
This format matches all the subdomains under the example.com domain, for example, a.example.com, a.b.example.com.
Reject for missing PTR: Reject a message when its sending IP address has no PTR record.
Reject for invalid PTR: Reject a message when its sending IP address has a PTR record, but for the PTR record, there is no mapping A record, or the IP address in the A record does not match the sending IP address.
The reverse DNS validation rule appears in the list on the Settings tab.
To remove a rule, select a rule and click Delete. You can also select the checkbox in the table heading row to select all rules except the default rule and Empty sender rule, which you cannot delete.