I've searched the KBs here, and I guess I'm just not too sure what to look for, so here it is!
I run an Exchange 2007 server in a hosted environment. I have a mailbox called email@example.com. We send emails from a few different sources on the internet using this email address. These users are not MAPI clients, they just connect to our server via smtp, password authentication. When this user sends email as firstname.lastname@example.org, all goes through just fine, however, when they try to send it as someone else, it gets the 5.7.1 error, client does not have permission to send as this sender.
How can I configure this user to be able to send as any email address?