cal (local domain)
us.com (exchange inside domain) (user has no access to this)
autodiscover.site2.com (exchange outside domain) (uses main email)
autodiscover.site3.com (exchange outside domain) (users 2nd email)
when the domain user tries to send and email from site2 receives an email from site2 exchange server saying you do not have permission to send, but can send from site3 with out issue.
Server 2008 DC w/exchange 2007
Windows 7 Pro