An e-mail alias is a nickname or alternate name for an existing e-mail address.
For example, if your name is John Smith and your domain name is smith.com, you might setup an e-mail account (mailbox) called jsmith. The mailbox will receive e-mail sent to jsmith@smith.com. If, for example, you want your family to be able to send mail to john@smith.com and receive that mail in the same mailbox, john@smith.com is an alias of jsmith@smith.com. Other good examples of this are webmaster@smith.com, info@smith.com and sales@smith.com. Aliases are particularly useful when you want your web site to have the "big company" feel with many department addresses (such as accounting@smith.com), but wish to receive all of the department e-mail messages in one location. Your sender addressing a message to "accounting@smith.com" is never aware that the message will ultimately reach jsmith@smith.com.