First, of all if you dont want them to, remove those permission which seems like you have done.
Create a new OLK profile or repair and test again
Test with an online profile.
Download the OAB if being used via OLK.Sukh

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We have a second mailbox set up which is our "Support" mailbox.
The problem is when we reply to an email in this mailbox it automatically populates the 'From' field with the 'Support' address and we have to manually change it to our own address.
I've tried setting deny Send As permissions on the support mailbox but for some reason we're still able to send? I thought an explicit deny overrules any other permissions set?
I don't want any of my users to be able to Send As, or Send on Behalf of the support mailbox.
We're operating off an Exchange 2007 server with a 2008 DC.
Anyone have any ideas?
Thank you,

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First, of all if you dont want them to, remove those permission which seems like you have done.
Create a new OLK profile or repair and test again
Test with an online profile.
Download the OAB if being used via OLK.Sukh

There is an amazing pack of free network admin tools. click here to download it