A reverse DNS return the hostname of an IP Address. It works exactly in reverse to DNS. When you send an email to someone, and they want to assure the mail is coming for an authorized mail server for your domain. Receiver AntiSpam engine in SMTP server runs a reverse DNS against the mail server addressed in initial SMTP banner.
For example, I have published an Exchange Server with public DNS record mail.m365proguide.com (Common namespace). But my SMTP banner address is the name of the Exchange Server EXC1.m365proguide.com. In this case, it will fail, if someone tries to reverse DNS to the EXC1.m365proguide.com as I don't have any record for this address. It scores a reason for the mail can be flagged as suspicious and sent to your recipient folder or rejected.
So, let us learn how to change SMTP banner of your mail server and verify it.
Go to https://mxtoolbox.com/ and find your mail server address
2. Click on the Test mail server option.
As you can see in the below picture, The SMTP banner announced is EXC1.m365proguide.com. But my actual mail server address is mail.m365proguide.com. As EXC1 is the local hostname of Exchange Server and it will result in reverse DNS failure.
3. Run below command from Exchange management shell to update SMTP banner
4. Test using MXtoolbox
5. You can also test using SMTP commands
telnet mail.yourdomain.com 25
