Alternatively, if an user adds your domain or email address to their "contacts" or their "safe-senders list" they will no longer see this notification.
If that doesn't work, try connecting directly to the IPs.
If you are able to connect directly to the IP and not mail.hotmail.com, then it is likely there is an issue with your DNS server.
This change is part of Microsoft's overall Trustworthy Computing Initiative and was made to further reduce the risk of malicious HTML content reaching our users.
One way to ensure that your messages aren't marked as being from an "unknown sender" is to join Return Path's Certification program, a third-party accreditation and reputation service that provides with a list of responsible senders.
This section provides troubleshooting information for senders who are having trouble reaching users by email.