that's sort of complex.
your machine has a 'domain name' --- a fake one. sort-of.
a domain name that we all know and love is www.whatever.com
or something like that. but your linux box can also have a domain name that does not have to be registered or real or anything. it is kind of like a 'workgroup' in windows. when you set up your box, it'll ask you what domain the machine is in, so give it some name like home.net --- so YOURNAME@home.net
email will get bounced to your home lan box. problem is, you have to setup a DNS entry on your local machines that tells them all to use 'home.net' as your local IP address rather than the real home.net.