Previously I knew how to test for traditional Open Relays on mail servers – but was looking for some more extensive testing and stumbled across this site:
http://www.dsbl.org/relay-methods
Among the list is methods of testing against double bounce and webmail relaying….
In addition to this the base-64 encoding and decoding tool can be used to test SMTP Auth on servers:
http://legacy.dillfrog.com/tools/base-64_encode/
Very useful 