I just noticed this issue today. User A can send email as some other users (not all users) without "Send As", "Full permission". (i.e. A colleague can send an email as my account to me. I'm sure he does not have permission on my account.
Does anyone know if there is an known issue in Exchange 2010 SP1 RU5?
I'm still working on the cause.
Thank you,
JohnnyW2012