Email addressing varies.
How you do it on your computer and your network might be different
than what I show here.
Basically, though, there are two kinds of email addresses:
If you're sending a message to someone who has an account on your computer,
address it to that person's username (login name).
For instance, if Marty Waters logs on to his account with the username
mwaters, you can address his mail to mwaters.
If you're sending a message to someone who reads mail on another computer,
you'll usually need to include that computer's name in your address.
Two forms of addresses are pretty common:
Internet addresses have a username, followed by an at sign (@) and the
other computer's hostname.
For example, if Edie Nisbaum's username is edie and
her computer is named giraffe.zoo.utx.edu you'd
address Internet mail to her as edie@giraffe.zoo.utx.edu.
UUCP addresses have the other computer's name, followed by an
exclamation point (!) and then the username.
If Sandi Shore is sandis and her computer is named beach,
you might address her mail as beach!sandis.
Those address guidelines won't be right for everyone.
If you have questions, ask your system postmaster or administrator.