Your IP doesn't seem to be listed at public black lists,
http://mxtoolbox.com/SuperTool.aspx?action=blacklist%3A5.9.79.211
Your domain proteus.otetto.org resolves nicely (and back), and as far as
I could see, SPF is fine too.
It appears that they have blacklisted your IP address or a netblock
containing your address. How long have you had this address? Is it
possible that the previous owner was a spammer (or had his server taken
over by spammers)?
I'm afraid there is not much you can do apart from asking Microsoft
nicely to unlist your IP or you need to change IP address altogether...
Failing that, could you try connecting over IPv6 to that server and see
if this works? If that works and you fancy the extra configuration you
could manually force a IPv6 connection for certain domains (not even
sure this is configurable).
Thomas