Once you have logged in open another browser tab and go to https://postmaster.live.com/snds. The useful links are –
View Data: summary report on host IP addresses with historical data going back possibly months.
View IP Status: gives a list of any IPs that are blacklisted.
Request Access: allows you to add IP addresses or address ranges to be included in the reporting.
The other links should be self-explanitory.
If one of the IPs is blacklisted go to this link http://go.microsoft.com/fwlink/?LinkID=614866 and fill out the form. You will usually get an automated response fairly quickly but it’s really up to them how soon they take any action. In recent times it has only been a few hours.