I was suffering the same problem about 7 years ago on my own personal mail server. I discovered that if I padded out the message headers with a load of junk headers to push the message size up beyond about 50KB, then the otherwise exact same message would arrive in the Inbox at Hotmail rather than being blackholed.

https://lists.exim.org/lurker/message/20070614.130838.42d1bd...

I haven't done this for many years though as it was such a terrible hack. I do wonder if it still works, or if the threshold has changed though.