In this guide, we delve into one of the common conundrums that mail server administrators on CentOS 7 may face: the error message "450 4.7.1 Client host rejected: cannot find your reverse hostname" when sending emails using Postfix. This issue is typically associated with DNS configuration and can be resolved by correctly setting up the reverse DNS record (PTR).

Introduction to the Issue

When sending an email, the recipient's mail server may verify the sending server using reverse DNS lookup. If there is no corresponding PTR record set up for the sender's IP address, the recipient's mail server may reject the connection with the error "450 4.7.1 Client host rejected: cannot find your reverse hostname". To address this problem, we need to ensure that a proper PTR record is set up for our public IP address.

Step 1: Verify the Existence of PTR Record

Firstly, we need to verify whether a PTR record exists for our IP address. This can be done using the dig or nslookup tool.

Example using dig:

dig -x your_public_IP_address +short

If the command does not return a domain name, the PTR record likely does not exist.

Step 2: Request Setting up PTR Record

PTR records are typically managed by your internet service provider or hosting company that allocated you the IP address. You will need to contact your provider requesting the setup or update of the PTR record for your IP address to reflect your mail server's domain name.

Step 3: Postfix Configuration and DNS Records

After setting up the PTR record, it's important to ensure that the domain name used in the PTR record matches the name your mail server announces when initiating SMTP connections and also matches the name specified in the DNS A record of your server.

In the Postfix configuration file /etc/postfix/main.cf, set:

myhostname = fully_qualified_domain_name_of_your_server

And ensure that the domain name has a corresponding A record in your DNS zone.

Step 4: Restart Postfix

After completing the configuration, restart Postfix to apply the changes:

systemctl restart postfix

Correctly setting up the PTR record and maintaining consistency between the PTR record, the name announced by the server, and DNS records are crucial for successful email delivery and preventing errors like "450 4.7.1 Client host rejected: cannot find your reverse hostname". If issues persist, we recommend further diagnostics using SMTP communication logs and consulting with your internet service provider.