E-mail from outlook.com is having issues right now because at least 750 of their servers are infected with or affected by some kind of botnet malware.
Our servers are equipped with fail2ban which watches logs for abusive behavior and locks those IP addresses out for a period of time. The attacks are still ongoing so releasing these blocks do no good and potentially harmful if malware were to spread to our servers or our customers machines.
I have notified Microsoft of this via their tech and abuse e-mail contacts and I have forwarded a list of the 750 servers thus far affected. This can affect other domains as they contract with Microsoft for mail delivery.