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.
In addition, senders who are on the Return Path Certification list or on a user's "safe sender's" list typically experience links and images within their messages enabled by default. An "allow list" is essentially a "free pass" which allows emails from certain senders to bypass junk email filters and other precautions. You can find out more about our filtering processes here. who helps ensure the legitimacy of certain senders via their Return Path Certification program.
This is the practice of verifying email addresses without sending (or attempting to send) emails to those addresses.
This method is commonly used by malicious senders to generate lists of valid e-mail addresses that they can send spam, phishing emails or malware.
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.
Once the IP has built a reputation for not sending spam, will typically allow for a better email delivery experience.