Smart Screen® email filters are influenced by a number of factors related to the sending IP, domain, authentication, list accuracy, complaint rates, content and more.
Of these, one of the principal factors in driving down a sender's reputation and deliverability is their junk email complaint rate.
This is the practice of verifying email addresses without sending (or attempting to send) emails to those addresses.
In some cases legitimate senders advertise themselves incorrectly as a non-internet routable IP when attempting to open a connection to 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.Some of the deliverability issues are the result of sender-based software configurations.If you are running anti-virus software on your firewall or SMTP server, check for the setting "Internet Email Auto Protect" or "Internet Email Protection." If this setting is enabled, disable it and try sending a test message to our servers again. If you are unable to connect, then attempt to telnet over port 25 directly to our email servers (MTAs).
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.