After much toing and froing I did manage to get somewhere with Hotmail. At this stage I cannot switch it back on for DD usage as I’m still making investigations, but I did discover that the reason we were being blocked as spammers is because some kind soul discovered that they could use our Support form to send out e-mails to any address, not just the address we intended.
So in fact, although they wouldn’t tell me why, Hotmail were actually correct. My server was spamming.
The loophole is now closed. The closure of the loophole MAY cause you some issues, if it does you’ll see a Kabarty branded page saying you sent something to which the server objected. If you do see that, please fill in the details of what you were trying to do, where and when so that we can improve the system for you.