Exchange 2010 Get-MailboxJunkEmailConfiguration/Set-MailboxJunkEmailConfiguration cmdlets.

Turing on the Junk Folder for the Yaniv mailbox is simply a matter of entering this cmdlet at the Exchange Management Shell:

Set-MailboxJunkEmailConfiguration -Identity Yaniv -Enabled $true

to disable it:

Set-MailboxJunkEmailConfiguration -Identity Yaniv -Enabled $false

And to enable it for all users in the specified Mailbox Database:

Get-Mailbox -Database <Mailbox Database Name> | Set-MailboxJunkEmailConfiguration -Enabled $true

Successfully