How To Fix ‘With 554 No SMTP’ When your Mails Get Reject By GMX or other Mail-Provider

Summary

The article provides a solution for fixing the 'With 554 No SMTP' error when a private email server's emails are rejected by GMX or other mail providers, specifically focusing on the importance of setting the correct PTR (Reverse DNS) record for the mail server.

Abstract

The author of the article encountered an issue where their NextCloud environment was unable to send emails to a GMX address. Upon investigating the mail server logs, they discovered an error message indicating that the GMX mail server was refusing to communicate due to a '554 No SMTP' error. This error is often related to the PTR-Record not being correctly set to the mail server domain, which is a critical setting for spam recognition and adherence to SMTP standards. The article explains that the hostname in the SMTP greeting must have an A record pointing back to the same server, as per RFC821 4.3 and RFC2821 4.3.1. The author suggests using a reverse lookup tool to check the PTR-Record and provides a personal example of how they resolved the issue by updating the PTR entry to match their mail server's URL through their server's admin interface. The author emphasizes the importance of pointing the PTR record to the mail server's specific URL rather than the 'www' domain.

Opinions

The author believes that the PTR-Record is a crucial factor in email delivery success and spam recognition.
The article implies that adherence to SMTP standards, such as proper DNS configuration, is essential for reliable email service.
The author points out that forgetting to set the PTR record correctly is a common oversight, even for those who are knowledgeable about mail server setup.
The author suggests that checking the PTR record should be a standard troubleshooting step when encountering email delivery issues.

A private email server often has problems with a public email provider…

Yesterday I had a problem that my NextCloud environment can’t send E-Mails to a user who has an GMX address. I searched the mail server logs and found this entry:

postfix/smtp[xxx]: to=<[email protected]>, relay=mx00.emig.gmx.net[212.227.15.9]:25, 
delay=139120, delays=139120/0.01/0.07/0, dsn=4.0.0, status=deferred 
(host mx00.emig.gmx.net[212.227.15.9] refused to talk to me: 554-gmx.net (mxgmx011) 
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=my-server-ip&c=rdns)

I found out that this message will be produced if the PTR-Record (reverse DNS) is not set correctly to your mail server domain. For some mail providers, this setting is important, since spam recognition software and RFC821 4.3 (also RFC2821 4.3.1) state that the hostname given in the SMTP greeting MUST have an A record pointing back to the same server.

To check the PTR-Record use this site. For me it was something like this:

static.xxx.xxx.xxx.xxx.clients.your-server.de

After I found the problem I go to the admin interface of my server and changed the PTR (Reverse DNS) entry into the URL of my mail server:

mail.MY_EMAIL_DOMAIN.de

It is really important that the entry point to the specific URL of the mail server and NOT to the www domain!

BTW: I already mentioned this within another article but forgot to set it correctly:

Make sure that the PTR record of your IP matches the FQDN (default : mail.domain.tld) of your mailserver host. This record is usually set in your web hosting interface.