When using postfix within a Proxmox Container (LXC), Postfix might source a wrong domain and just appends .localdomain to the sender/receiver address. To change that edit /etc/postfix/main.cf and change the line mydomain = '': <code bash>mydomain = <yourdomain></code> If not changing that, .localdomain would be used although the ''/etc/hosts contains the correct domain.