Oh for fucks sake
Why oh why have Microsoft not added an option to Exchange* along the lines of:
"Require additional verification if 'To' field exceeds <xxxx> addresses [Y/N]"
that is "Y" by default ?
Memory is dim now (which makes it even worse) but I have a feeling it was kicked about last millenium when RFCs were being circulated.
*Other mail servers are available.