Mandrill and Hotmail Hard bounces....

I have just finished my first mailing with Mandrill.

I had a 260 emails/hour limit, then 500 per hour limit after the first batch,
It took almost the day to send my 3,400 emails.
I got a 99% delivery rate with 27 hard bounces.

What I find a bit strange is that 24 of the hard bounces are hotmail emails and a week ago I had no hard bounces at all when sending to the same list with my own server.

Got this kind of error message with some variations:
smtp;550 Requested action not taken: mailbox unavailable bad_mailbox
smtp;553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1) bad_mailbox

I checked the 27 bounces that are now black listed with and I got 18 OK and 9 Fail. I don't know what to think.

Has anyone noticed anything with hotmail bounces and Mandrill?