If this isn’t a phishing email itself, your email address was probably harvested from a compromised site you used it to sign up with. There are sites where you can check to see if it’s compromised. This is why I started using email aliases when signing up for any site or service. It shows where it was compromised or you’ll find some companies will share it with partners or sell your info sometimes.
I used to use Relay but they had gotten added to a couple of disposable email block lists and because of that started having issues with my accounts...
Idk if SimpleLogin has that same issue or if there's a way around the problem entirely
Not sure. But Proton, Apple, passmail SimpleLogin (got names mixed up) and some other providers have a way you can create email aliases on the fly that forward to your real address. I think Microsoft does too but it was limited last time I looked at it.