1. To send a mail message to a user on the same system:
jane
This is the address for the user jane on the same system as the person sending the mail.
2. To send a mail message to a user on another local system:
ezax@frolix
This is the address for the user known as ezax on another local system frolix.
3. To send a mail message to a user on another system in another domain:
helper@ubik.ed
This is the address for the user helper on the system known as ubik, which is in the domain known as ed (for edinburgh)
4. To send a mail message to a user on another system which is located a higher domain:
ivan@islay.glas.com
This is the address for mail to the user ivan on another system, islay. This system is in the domain of glas (for glasgow) which is itself in the domain known as com (for commercial).