Investigating Gmail’s “This message seems dangerous”

I’ve been receiving multiple user reports that Gmail shows a red “This message seems dangerous” banner above some of the emails sent by Healthchecks.io. I’ve even seen some myself:

Gmail’s “This message seems dangerous” banner in action

The banner goes away after pressing “Looks safe”. And then, some time and some emails later, it is back.

It’s hard to know what exactly is causing the “This message seems dangerous” banner. Google of course won’t disclose the exact conditions for triggering it. The best I can do is try every fix I can think of.

Here’s how I fixed it: read the full article at blog.healthchecks.io.