"Send As" allows one user to send an email as though it came from another user. The recipient will not be given any indication that the email was composed by someone other than the stated sender.
"Send As" can only be granted by a system administrator. "Send on Behalf of" may be more appropriate in many situations, it allows the recipient to be notified both who the author was and on who's behalf the email was sent.
The following procedure will allow system managers to grant users the ability to send as another:
Log onto the server running Exchange.
Run Active Directory Users and Computers.
Under the "View" menu ensure that "Advanced Features" is ticked.
Find the user's account that you want to be able to send as, and open up the account properties.
Select the "Security" tab.
Click [Add ...] (under "Group or user names") and add the user (users or group) that is to be granted permission to send-as this account.
For each account added, highlight the account under "Group or user names" and in the "Permissions for ..." window grant the account "Send As" permission.
Click [OK] to close the account properties dialog.
Note:
If there is an account for which a number of people need to be able to send as (such as an account used as a single point of contact for a distribution lists) then administratively it may be simpler to add a group of users who should have that permission and grant the permission to the group and not to the accounts individually.
The process of sending an email as coming from another account is the same as sending on behalf-of. See How to Send a Message on Behalf of Another for guidance notes.