No Send As permission in Server/Exchange 2003?

I’ve been having problems with a particular user who cannot send email from our new SMTP server unless he’s in the Domain Admins group. I’ve read in other posts about there being a “Send As” permission to set somewhere explicitly, but when I get into the mailbox rights, no matter where I look I don’t see a permission that is called Send As. I see others, such as full mailbox control, delete mailbox and so on, but nothing in the advanced tab or otherwise is there a permission called Send As. Is this normal? Am I looking in the wrong place?