Ok, thanks for the pointers.
"Another solution would be to get a multi-domain SSL cert, but you need to get a new one each time you want to include a new domain. "
that's what I thought too.
"Since the SMTP protocol does not tell the server what domain name the
client thinks it is connecting to, there is no way to do what you ask. "
I'll test multiple instances setup > need to get a test rig running 1st, I'll let you know how that went ... > future inclusion in ISPconfig ? > every domain running it's own instance
Windows, the only virus you pay for