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I'm running Postfix 2.10.1 on the same server as my webserver so the domain is example.com and it is receiving mail locally but not from any external networks. The server is running CentOS 7.

For the DNS, I'm not exactly sure the proper settings. I currently have an MX record for example.com and a default A record with the IP of the server. All of the resources I found online were using a subdomain like mail.example.com and then had a corresponding A record. I've tried it both ways, where the A record points to the same IP address, but it didn't seem to make a difference.

For the Postfix configuration, master.cf wasn't changed and I made the following changes to main.cf:

myhostname = example.com
mydomain = example.com
myorigin = $mydomain
inet_interfaces = all
inet_protocols = all
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain
mynetworks = 127.0.0.0/8

The following TCP ports are also open: 25, 465, and 587.

Additionally, I was also curious about mail_owner. None of the resources I read mentioned it or creating a user for Postfix. Should I create one, and if so, what files/directories should it own, if any?

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There are a few issues that may be causing your connectivity issues:

  • Postfix may not be listening on your external address. 127.0.0.1 does not connect externally. Try the command netstat -ant | grep LISTEN to see what address Postfix is listening on.
  • Your firewall may not be forwarding the necessary ports to your server. Instructions differ depending on the device you are using.
  • Your ISP may be blocking traffic on port 25 (SMTP). Port 443 (SMTPS) was deprecated, and is not commonly used. Port 587 (Submission) should only be used by your devices to send mail from your domain. Port 587 is likely not blocked.

There are a number of sites on the web which will check connectivity to your server.

As someone who has been running a couple of domains, I do have comments on domains for mail servers. Domains like example.com or www.example.com are almost certainly sending spam and increase the likelihood your mail will be classified as spam. (It really doesn't matter for am MX as I would be sending mail to you and trust my senders are not sending Spam.)

If you are sending mail from the domain, mail will be more likely to get delviered if:

  • You have a static IP addrees;
  • You have use a sub-domain other than www for you mail traffic (typically SMTP or MAIL);
  • You have a pointer record for the mail sub-domain (this will likely need to be configured by your ISP);
  • You have an SPF record for your domain (sender addresses should not have a subdomain);
  • You have an SPF record for the sub-domain your mail server is sending as;
  • You use DKIM, with the key published to DNS, to sign your outgoing messages; and
  • You have a DMARC policy record in DNS.

If you don't have a static IP address, use your ISP's relay server and configure your SPF accordingly. There is a risk your ISP's relay server might get blacklisted.

The standard mail_owner user id is mail, although others are used. This is used to receive mail and other activities that don't apply to a specific user. Usually this userid owns mail received for `root. Most installers will ensure the userid exists.

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