Had a client today stop receiving inbound emails on their SBS2008 server. It turned out that Windows Update Services – WSUS - had maxed out the hard drive! 
So I moved the Exchange database to a new spare drive, as explained so clearly here:
http://www.b4z.co.uk/tag/4-3-1-insufficient-system-resources
and cleaned up WSUS, which is explained clearly here:
http://blogs.technet.com/b/gborger/archive/2009/02/27/what-to-do-when-your-wsuscontent-folder-grows-too-large.aspx
Thanks guys! 
Scribbles Out.