If you find you are unable to send email to Outlook or Hotmail addresses, check your mail server log. Entries like the follow mean that your server has been blacklisted:
550 SC-001 (BAY004-MC4F49) Unfortunately, messages from 18.104.22.168 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.
Please note that by "Internet service provider", Microsoft actually mean the SMTP server operator - i.e. you and/or your system administrator.
Like many customers who encounter this issue, you are likely surprised since you do not send spam. Unfortunately we find that Microsoft's Outlook/Hotmail service is rather prone to blacklisting senders where the number of received emails from a particular IP is low.
While the internal mechanisms are only explained in very generic terms, essentially Microsoft assigns every IP address a "reputation". This server reputation (or trust) has to be earned over time; where an IP sends little or no email to Microsoft, the default reputation is low and its common to reach this error state.
You can request "delivery mitigation" (i.e. removal from their IP blacklist) here:
You may also like to join their "Smart Network Data Service" (SNDS) program, which can alert you in the future if you are re-listed and sometimes will provide additional information about why the IP has been listed.
To do so just enter your IP here:
https://postmaster.live.com/snds/addnetwork.aspx (Microsoft account required)