Both valid points. I think the email scanner issue will be especially common. Could you explain more how the Slack browser token would work? Warning the user not to open the link if they weren't expecting it is a pretty common approach.

My initial plan was to require the user to reply to the email, which solves the scanner problem and has the benefit of not opening a new tab. This comes at the cost of being an unfamiliar flow, and requiring an extra click. What are your thoughts on that?

I ended up starting with links since it worked for me (apparently gmail and fastmail don't pre-open links) and was simpler to implement for the prototype. Also it's less dependent on the speed of the user's email server.