I register my domain at 1and1.com, let's say it is abc.com. Then, I start an aws ec2 instance, with ip address 220.127.116.11.
At 1and1 dns control panel, I created a A record to point abc.com to aws server ip address 18.104.22.168. So far so good, I open abc.com will display the right content which I setup at 22.214.171.124 web server.
In the meanwhile, I use the free 1and1.com webmail service(so someone send to me firstname.lastname@example.org, I will receive the email via 1and1 free mail server). Also, I need 126.96.36.199 web server to send notification emails to user represent my abc.com site. Thus, I install postfix in 188.8.131.52 for send out email only.
The trouble is some public email providers will reject emails from 184.108.40.206.
I checked the log
(host mx01.mail.com[220.127.116.11] refused to talk to me: 554-mail.com (mxgmxus006) Nemesis ESMTP Service not available 554-No SMTP service 554-Bad DNS PTR resource record. 554 For explanation visit http://postmaster.gmx.com/en/error-messages?ip=18.104.22.168&c=rdns)
For example, the below is the typical answer I asked them why rejecting my email sent from 22.214.171.124 since it is not a spam.
Your mailserver ip has still the generic default rDNS / PTR-RR from the beginning:
$ host 126.96.36.199 188.8.131.52.in-addr.arpa domain name pointer ec2-1-2-4-7.ap-southeast-2.compute.amazonaws.com.
Please setup an valid und speaking mailservername in the rDNS / PTR-RR.
Why not mail.domain.tld?
To be frank, I've no idea what they are talking about and I have no clue what shall I do either. Could anyone enlighten me? Thank you.