Whilst on the face of things mail.app in Snow leopard should work out of the box with exchange 2007 it still needs settings changing, in this case to send attachments over 5meg. The following fixes this:

You need to edit web.config in the following 2 files and change maxrequestlength to a size in kilobytes i.e. 50000 for ~50meg:

C:\Program Files\Microsoft\Exchange Server\ClientAccess\exchweb\ews
C:\Program Files\Microsoft\Exchange Server\ClientAccess\owa

Then also from a command prompt do the following:

cd \Windows\System32\inetsrv