OK, didn't have any luck searching for this problem, but on browsing came across a thread that indicated this is a problem with DNN 5.6 and a couple of Data Springs modules.
I've tried the email validation fix, and it doesn't appear to fix the problem but if I remove the FROM text from a newsletter it DOES go.
So, is there an update for this? Also, I also run Dynamic Registration and Dynamic Forms.. updates for them too??
Pointers appreciated