Originally Posted by kapasaca
most likely now is that the permissions are off.
check AD and open up the affected user, tab security and see if the BESAdmin user is in there.
It should have the "send as"checked as allow on user objects.
futhermore you could check if the affected user is member of an admin group.
They had SendAs, and they are not admins. I know what happens on Admin accounts, we ran into that a few years ago I think. Besides, they could send an e-mail to a gmail account but not one to inside the domain. They started working again without us doing anything.