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I register my domain at 1and1.com, let's say it is abc.com. Then, I start an aws ec2 instance, with ip address 1.2.3.4.

At 1and1 dns control panel, I created a A record to point abc.com to aws server ip address 1.2.3.4. So far so good, I open abc.com will display the right content which I setup at 1.2.3.4 web server.

In the meanwhile, I use the free 1and1.com webmail service(so someone send to me admin@abc.com, I will receive the email via 1and1 free mail server). Also, I need 1.2.3.4 web server to send notification emails to user represent my abc.com site. Thus, I install postfix in 1.2.3.4 for send out email only.

The trouble is some public email providers will reject emails from 1.2.3.4.

I checked the log

(host mx01.mail.com[74.208.5.22] 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=1.2.3.4&c=rdns)

For example, the below is the typical answer I asked them why rejecting my email sent from 1.2.3.4 since it is not a spam.

They said

Your mailserver ip has still the generic default rDNS / PTR-RR from the beginning:

$ host 1.2.3.4 1.2.3.4.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.

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Many MXs check a connecting host for proper setup. Make sure you've set up

  • meaningful PTR for reverse DNS
  • A record for that PTR (FCrDNS)
  • meaningful HELO FQDN host name (must resolve to IP)
  • reasonable security that your host isn't an open relay
  • and don't send spam so you get listed on a DNSBL

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