Confirmation mail for forum sign up: sending side configured wrong


I tried to sign up for this forum which needs me to click a link in an confirmation email sent to me.
However, that mail never reached me.

As I run my own mailserver the postfix logs told me why:

Jun 4 14:03:10 mail postfix/smtpd[27647]: warning: hostname does not resolve to address Temporary failure in name resolution
Jun 4 14:03:10 mail postfix/smtpd[27647]: connect from unknown[]
Jun 4 14:03:11 mail postfix/smtpd[27647]: NOQUEUE: reject: RCPT from unknown[]: 450 4.7.25 Client host rejected: cannot find your hostname, []; from=<cosmos1+verp-585d7[censoredforprivacy]> proto=ESMTP helo=<>

My mailserver is configured to


The only chance to receive that confirmation mail was to temporarely deactivate the option(s) above.

As every good mailserver should have those options turned on I suggest changing something on the sender side. Maybe a lot of people can not sign up here because of that.