Mailbox loses send as permissions after a few minutes.
In Active Directory, go to Security tab, Advanced and set Default permissions.
This issue can be caused by the user account being a member of Domain Admins.
This article explains about Protected Groups & AdminSDHolder in further detail: http://support.microsoft.com/kb/907434h
Thursday, 7 July 2011
Exchange 2003: Unable to Access Mailbox via OWA
If you're unable to access a single mailbox via OWA check under Active Directory > Exchange Advanced > Mailbox Rights that SELF has Full mailbox access ticked.