Just FYI...
Hotmail accounts are spam-cursed. Microsoft will harvest the addresses that you send mail
TO even if you have opted out of their 'direct' spam list. Isn't that clever? You opted out of
receiving spam, but they didn't ask if you wanted all recipients of
your emails to be spammed. This is Bill Gates at his slimiest. So, whenever you send an email to someone from a Hotmail account you are pointing a gazillion spammers at that account.
Did anyone working with you on that project use a Hotmail account?
Other ISPs might have similar 'gotcha' policies, of course.