We had the problem that Microsoft servers rejected emails from our hMailServer. We got the following replies when we send a mail to @outlook.com or @live.com:

Remote server replied: 550 SC-001 (COL004-MC6F13) Unfortunately, messages from <our IP> weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors.

After looking for the error code and possible causes, it seems like spam has been sent via our IP, but none that we know of. It could be that it did not originate from our server, but from a server with an IP address starting with the same digits as ours (a whole range belongs to our server hosting provider).

How did I fix it:

  1. Checked if other services think that spam is being sent via our IP:
    1. https://www.spamhaus.org/lookup/
    2. http://mxtoolbox.com/blacklists.aspx
    3. https://www.senderscore.org/lookup.php?lookup=0
  2. Contact Outlook.com to let emails from our server through: contact form. After replying to their mail, our emails were not rejected anymore after a few days.
  3. Monitor emails going out from our server to Microsoft/Outlook: We signed up for Smart Network Data Service. There we added our IP, signed the contract, and will now be notified of spam and junk sent from our server.

Our emails now seem to land in the junk email folder, but at least they are pass through. We will look into the problem to persuade Microsoft to not mark our emails as junk.