Hello,
I'm using SME 6.0.1, I am only able to send emails to users of my domain. I am able to receive but everytime I try to send I get an error saying
"Sorry, that domain isn't in my list of allowed rcpthosts"
That just means that your server is not an open mail relay, which is a good thing.
You should either:
1. Connect to your server using a VPN client, then send mail as though you were connected on the local network of the server
or
2. Use an SMTP server which is close to you to send mail, rather than your server. For instance, use your ISP's mail server.
You're not located at the site of your server, right? You're trying to connect to it via the Internet and send mail that way? That's not the way it's designed to be used. When you are away on vacation, you send postcards via a postbox where you are vacationing. You don't (somehow) send them back to your house, and get someone to drop them in the local postbox near your house. Similarly here, you should be sending mail via an SMTP server near where you are, not where your server is.
Or 3. Use webmail.