Every user has a primary email address, which is the email address created when the user's account was created and the address used to log into Google Apps. Google Apps enables to you create additional email aliases for a user. See Adding email aliases for instructions. See Relationships and behavior for caveats to using email aliases.
An email alias is an alternative email address that points to an existing user account. For example, the user firstname.lastname@example.org might also want to receive email addressed to email@example.com. If you create firstname.lastname@example.org as an email alias for email@example.com, Google Apps will deliver email for both of these addresses to the same Gmail inbox.
If your Google Apps account has multiple domain names associated with it, a user's email aliases can include email addresses in other domains that are part of your account. For example, firstname.lastname@example.org could have the email alias email@example.com — as long as someone else does not already have that email address. You cannot reuse an email address that is already assigned.
You can create up to 30 email aliases for an individual, not counting the email aliases created automatically when you add domain aliases.
Sending email from an email alias
Creating an email alias automatically allows the user to receive email at the email alias address. If the user also wants to send email that uses the email alias address in the From field, he or she needs to set up a custom From address in Gmail. See Sending mail from a different address for the required steps.
Email aliases and domain aliases
If you have a domain alias for your primary domain, every user and group with an email address in the primary domain (firstname.lastname@example.org) automatically has an email alias address in the alias domain (email@example.com).