I have Postfix installed. I am doing some simple tests. I am trying to send an email from my account on a box from within my LAN. I am doing like this:

# mail username@domain.com
Subject: Test Test
lskdjfl
.

I then hit enter. Right after I do this, I receive an email in my inbox that says:

I'm sorry to have to inform you that your message could not be
be delivered to one or more recipients. It's attached below.

How do I fix postfix where it can send email to outbound receivers outside of my LAN? It works inter-box when I send it say root to username.

Thanks.

Open /etc/postfix/main.cf with an editor and edit the line;

relayhost and enter relayhost = [outmail_address_of isp_host]

This is the address that you use to send mail from your mail client which will be in mail settings and provided by your isp. The address is enclosed in the [ ] brackets as above.