Exchange 2007 users able to send emails from other accounts

I have a problem on Exchange Server 2007 running on 2008 R2 where users are able to send emails from other accounts by clicking on the from box. I have checked the permissions in exchange and user accounts as well as delegations  and made sure they are not in any admin groups. any ideas would be grateful.

Thanks
somatthiasAsked:
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Mahesh SharmaCommented:
Check if everyone is given full permissions:

Get-MailboxDatabase -Identity <DB> | Get-AdPermissions
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somatthiasAuthor Commented:
Have checked not a problem
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Simon Butler (Sembee)ConsultantCommented:
Send As is the permission that is doing this, not full mailbox.
Somewhere they have the permission.

Check what groups have Send As permission within Exchange and then track back to see if Domain Users or something has been added to the group.

Simon.
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Md. MojahidCommented:
You grant User B Full Access Permission for User A’s Mailbox.

Refer
 http://blogs.technet.com/b/ehlro/archive/2012/04/06/full-mailbox-access-rights-send-on-behalf-send-as.aspx
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Simon Butler (Sembee)ConsultantCommented:
Full mailbox doesn't grant Send As permissions.

Simon.
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Seth SimmonsSr. Systems AdministratorCommented:
This question has been classified as abandoned and is closed as part of the Cleanup Program. See the recommendation for more details.
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