Well a ticket to help desk sorted this out fairly quickly. So thanks and full marks for that.
And of course it turns out that there IS a filter file on the servers which blocks mail with supposedly dodgy content and it WAS blocking anything with 'sex' in it, even within another word. But thats sorted now.
I did raise the question on the help desk as to who is able to vet this file - because I feel a bit helpless not knowing what else might be in that filter which may even at this moment be blocking something innocent for my mail or those of all my customer accounts, due to similar oversight of the programmer ...
... but no reply yet.
regards
Chris.