Only when sending mail with an attachment, in this case only 15MB. (Users have no limits defined (limit boxes are un-checked) -- System wide limit is set at 30MB in Org Config -> Hub Transport -> Transport Properties for both send and receive.)
Queue viewer on the 2007 Exchange Management console shows the messages sat in a queue to.......
cn-first routing group,cn=routing groups,cn=first administrative group,cn=administrative groups,cn=domain, etc etc
with a delivery type of SmtpRelayToTiRg status retry.
User gets a 'delayed for 4 hours' message and then a final NDR when the delievery time expires.
Exchange 2007, running on Server 2008 sp1