I don’t use Simplelogin so I don’t know how their service works. The domains used for aliases don’t need to originate email so there’s no need for an SPF record. The A record for the hostname used for the MX record(s) could technically point at multiple IPs that could be changed often.

I own a significant number of domains and manage my own servers. There’s quite literally no way for anyone to prevent me from using an email alias.

My point was that trying to block email aliases is a fool’s errand. It’s a slight hindrance to only the least technical users. The entire 7c/fakefilter project is an exercise in futility.