That's not true either. I don't know how it's set up other than that we use exchange, but on our internal network, you can send an email to someone's firstinitiallastname (the part that is before the @ for external senders) and it will go through.
So on our production servers, I need the @, but on our dev/text servers, I don't. And no, the domain is not appended to the address before sending. That's the part I actually know about.
It's common practice to allow unqualified emails addresses (ie. without the @domain.tld) from local sources since it's trivial to look up against your own user list. Depending on the MTA, the domain may or may not be appended, but often the local organisation's default domain is implied. However, this shouldn't work when routing mail via external MTAs that have no knowledge of your organisational structure.
So on our production servers, I need the @, but on our dev/text servers, I don't. And no, the domain is not appended to the address before sending. That's the part I actually know about.