OK, so sending works fine, but what I receive is from "Username@ComputerName.localhost", which clearly cannot be replied to. Is there an IP address linked to the email that can be used to respond? -- I'm guessing the sender information doesn't just disappear into the ether.
No. Unless you are running a mail server such as Sendmail or Postfix and have a hostname and address registered on internet DNS, there is no way for the mail to find it's way back to you.
You could set this up and register your server with DynDNS or another service, you just have to be careful you don't become an open SPAM relay.
It's also possible to make the return address point to a valid eMail address you have, such as GMail.