Forum foundry apparently uses sendgrid when sending emails, likely to have their own data on who clicks on them, how useful it is, etc.
Outlook has a habit of viewing sendgrid links as spam/phishing unless they actively know it isn't. I'm not too sure the official reasoning, but logically it makes sense that clicking on a link that then gathers data might be an issue. The link itself (at least in this case) is safe, it's something that @John G or @dascrow may have to look into further.
For what it's worth, I just tested with gmail, and did not get any alert about the link being unsafe.