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=<firstname.lastname@example.org>, relay=mx00.emig.gmx.net[220.127.116.11]:25, delay=139120, delays=139120/0.01/0.07/0, dsn=4.0.0, status=deferred (host mx00.emig.gmx.net[18.104.22.168] 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:
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:
It is really important that the entry point to the specific URL of the mail server and NOT to the
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.