500 Internal Server Error – The target principal name is incorrect

If you're running SBS and using a self-signed certificate, make sure the name of the certificate matches the external address of your SBS, eg. mail.mundy.com.au (eg. not mundy.local). Also make sure the publishing rule in ISA matches this name, as the SBS wizard usually puts a different address, eg. publishing.mundy.local (change it to eg. mail.mundy.com.au).