The username and domain are always separated by the at sign or "@" symbol. Which, by the way, evolved from the phrase "at the rate of" in accounting and commercial invoices, but that's a whole other fish to fry...
If there is a subdomain, it may look simialar to this: username@sub.domain.com
If there is no subdomain in the mail server/computer network it may look like this: username@domain.com
And then, depending on the level of complexity of your organization's email directory, there may be other conventions in the username, or 'local-part' to differentiate users like this: joe.username@domain.com