Setting up a Barracuda Spam Filter

It is hard for me to test this one, but I think you need the parens to group the !useraccountcontrol and the or block.

(&
 (
  (!userAccountControl:1.2.840.113556.1.4.803:=2)
  (|
        (sAMAccountName=${recipient_local_part})
        (othermailbox=smtp:${recipient_local_part}@ourdomain.com)
        (proxyaddresses=smtp:${recipient_local_part}@ourdomain.com)
        (mail=${recipient_email})
        (userPrincipalName=${recipient_local_part})
  )
 )
)

You should be able to do the following:

From the Exchange Management Console, under Org config -> Hub Transport, On the Actions pane select New Transport Rule. Give it a name and remember to select the Enable box.

Select "when the size of any attachment is greater than or equal to limit" and set this to your delivery limit.

Select "send bounce message to sender with enhanced status code" and set the message to something that communicates the acceptable limit.

Finish and restart the Transport service.

There's a full walk through here.