I am saying "Thanks you" to the +Net support staff who have sorted out their problem that I encountered. I was not happy to wait for 15 minutes, times 3 calls, but c'est la vie - they were no cost to me calls. No one actually knows why all my outgoing and some incoming mails were rejected as "Spam" Any great service and well done all.
I can see that our Networks Team raised a case with our Mail Team who have resolved the issue regarding the signature that was relating to a particular domain. As this has now been reset, you should start to see an improvement within a couple of hours.