Safe Links (also known as URL Protection) are tools used by anti-SPAM technology solutions that rewrite all incoming URL’s in email to point to the anti-SPAM provider’s proxy instead of the original and potentially dangerous URL in the recipients email. This allows all email to flow quickly into recipient inboxes while also allowing SPAM providers to check the safety of the website links in question.
If the Anti-SPAM provider finds the website behind the link it removed and rewrote to itself, is potentially malicious or appears to be a credential stealing website, it will NOT proxy connections to that website when clicked by email recipients. Instead it will present a message that warns the end user that the website behind the LINK appears to be malicious and not to proceed. Some technology solutions allow the end user to go anyways (at the discretion of the Administrator of the “Safe Links” service). While others just block access entirely.
Additional Reading:
Microsoft Advanced Threat Protection (ATP) Capability – Safe Links