Same situation as some time ago: A user on one of my domains, hosted on my server enables the autoresponder function within http://mydomain.tld:81/mailuser. The autoresponderfunction itself works as it should as it performs an automatic reply and the user receives the email which has been sent its emailaddress. So, all works great. However, when an email has been sent from an emailaddress, which is not on my server, the sender of that email receives an error from its ISP, like: Action: failed Status: 5.0.0 Domain name required for sender address username (in reply to MAIL FROM command) This means that I miss the @domain.tld (the right part within email@example.com) when the automatic reply has been sent. So I’ve checked the content of the following files: /etc/hosts 127.0.0.1 localhost.localdomain localhost 22.214.171.124 host.mydomain.tld host Within my /etc/postfix/main.cf I have the lines: myhostname = host.domain.tld mydestination = /etc/postfix/local-host-names virtual_maps = hash:/etc/postfix/virtusertable myorigin = /etc/mailname Within /etc/mailname I have the lines: host.mydomain.tld The output of hostname is: host.mydomain.tld And the output of hostname –f is: host.mydomain.tld I can not find any mistakes within my configuration, can anyone help me to find out, what I miss?