Microsoft Defender SmartScreen: "This site has been reported as unsafe"

When our Microsoft Edge users click the email verification link, Microsoft Defender SmartScreen pops up a giant “This site has been reported as unsafe” warning.

We’ve been unable to get any clear direction about this. Their general guidelines suggest that it might be a problem with SSL certs, but we haven’t been able to identify any issues there. Has anyone else run into this and/or been able to solve it?