Spammers, trolls, and ban evaders often use temporary email addresses. PieFed now checks a list of known temporary email providers and displays a warning icon next to registrations that use such services.
If registration mode is set to "Open" (no approval needed) then the site admin(s) receive a notification instead.
A throwaway email address isn't always a bad thing but it's one factor that admins might want to take into account.
I have to chime in and say this feels a bit underthought feature. I use a throwaway email for everything possible, and I would imagine a large portion of Fediverse users do that too.
I also get the motivation behind the feature. I didn't feel like throwaway addresses are worth it before I started using them. They may seem like an obvious spammer flag. But I'd say it's 50/50, just like with any free email provider like gmail or Proton mail.
We'll see. If it turns out to be useless noise, I'll remove it from the signal.