Cant't send email from a same network machine

[kilburn]

The problem is that your internal machine is using just the "host" part in the HELO smtp command, instead of the fully qualified domain name (host.domain.tld). There are three ways to solve this issue:

1. Setup your machines so that they use the fqdn instead of just the host part
2. Add your network to the mynetworks directive in the server's postfix main.cf file.
3. Make your clients use smtp authentication when relaying mails through the server.