Problem: email is being rejected by recipient server due to DNS mismatch
Solution: Confirm Reverse DNS is configured
Go to MXtoolbox.com and select SPF records
Then enter in ptr:(your external IP) for example ptr:220.127.116.11 as shown below:
Click “SPF Record Lookup”
And you should see the following with your domain name listed. If it is not Contact your ISP and ask them to create an RDNS record for you.
Reconfirm the RDNS record was changed and propagated by repeating steps above.