Outlook.com rejects: 550 SC-001 (BAY004-MC5F13)

I am operating a cluster of 3 dedicated machines for a webapp, each of those uses sendmail as MTA. Hostnames are node1.domain.com node2.domain.com node3.domain.com

Unfortunatelly outlook.com rejects to deliver mails to its users from my servers telling me the following:

X-Authentication-Warning: node1.domain.com: www set sender to [email protected] using -f

AND

Action: failed Status: 5.0.0 Diagnostic-Code: SMTP; 550 SC-001 (BAY004-MC5F13) Unfortunately, messages from 136.243.73.202 weren't sent. Please contact your Internet service provider since part of their network is on our block list.

This last message is strange as the ip does not apear to be on any blacklist I checked.

How could I fix the config regarding the authentication warning? I guess this is the main reason for the rejection. Thank you in advance for any help.