We have "USER A" who needs "send as" rights to "USER B" mailbox. In exchange 2007 management console, I right click on "USER B" account and clicked on Manage Send As Permission and added "USER A" to it.
Next day "USER A" couldn't send emails and when I checked Manage Send As Permission in Exchange Management Console, the name wasn't there.
I also set it from Active Directory and still same issue. Username gets removed after some period of time.
Does anyone know why this is happening?