Microsoft s'ammuse à bloquer des IP comme des cowboys (https://jeekajoo.eu/links/?-gJEew). Il suffit que tu sois sur un bloc d'IP sur lequel il y a une IP qui abuse, et ils interdisent à TOUT le bloc d'envoyer des mails à leurs serveurs.

Voici donc le formulaire pour se faire débloquer...
Pour ma part ils ont répondu en 30 minutes.

Dans le mail que j'ai reçu, on peut lire que le déblocage peut prendre 24 à 48 heures pour prendre effet:
"""
We have completed reviewing the IP(s) you submitted. The following table contains the results of our investigation.

Conditionally mitigated
195.154.216.148
Our investigation has determined that the above IP(s) qualify for conditional mitigation. These IP(s) have been unblocked, but may be subject to low daily email limits until they have established a good reputation.

Please note that mitigating this issue does not guarantee that your email will be delivered to a user’s inbox.

Ongoing complaints from users will result in removal of the mitigation.

Mitigation may take 24 - 48 hours to replicate completely throughout our system.

If you feel your issue is not yet resolved, please reply to this email and one of our support team members will contact you for further investigation.

Regardless of the deliverability status, Outlook.com recommends that all senders join two free programs that provide visibility into the Outlook.com traffic on your sending IP(s), the sending IP reputation with Outlook.com and the Outlook.com user complaint rates.

Junk Email Reporting program (JMRP) When an Outlook.com user marks an email as "junk", senders enrolled in this program get a copy of the mail forwarded to the email address of their choice. It allows senders to see which mails are being marked as junk and to identify mail traffic you did not intend to send. To join, please visit http://support.msn.com/eform.aspx?productKey=edfsjmrpp&page=support_home_options_form_byemail&ct=eformts.

Smart Network Data Services program (SNDS). This program allows you to monitor the ‘health’ and reputation of your registered IPs by providing data about traffic such as mail volume and complaint rates seen originating from your IPs. To register, please visit http://postmaster.live.com/snds/.

There is no silver bullet to maintaining or improving good IP reputation, but these programs help you proactively manage your email eco-system to help better ensure deliverability to Outlook.com users.

Thank you,

Outlook.com Deliverability Support
"""

D'autres références:
https://chabotsi.fr/links/?ld47fA
http://www.mypersonnaldata.eu/shaarli/?sy9Q9w
https://twitter.com/WikiNorn/status/595336279962865665