Hotmail has bounced many connections with a 'fatal' error code: 550 SC-004. This happened at about 0015 on 19 March (UTC).
Looking at Hotmail's error page at: http://mail.live.com/mail/troubleshooting.aspx#errors says it was rejected for "Policy reasons"
The result is many (all?) hotmail users have been 'disabled' in the mailing list.
List users can reactivate themselves at https://lists.wikimedia.org/mailman/options/wikimedia-l
Regards, Richard.