The simplest work-around would be for you to stop using hotmail when signing up for new forum accounts. At least until they get the issue resolved by hotmail. Either that or start a report to the hotmail team to have them stop blocking the emails from this domain.
I would highly suggest creating a gmail (google mail) account for now. IMO, it is far superior to hotmail in every way
Who knows, you might grow to love it
I have never once had email issues like this when using gmail, but I have seen similar issues on Hotmail multiple times, including for registering for my old SH forums before I moved to the current ones.