I can reply from ANY address from my domain including the exact one that was used to send me an email.
I can “deactivate addresses” by sending messages to a particular address straight to trash with rules.
Edit: turns out Fastmail has a masked addresses feature built in, separate from a catch-all. It’s basically simplelogin built in, if you want to enable it. Proton is looking more and more overpriced.
If I were a professional spammer, the first thing I’d do to clean the address list I have is to strip out plus addresses. It’s a simple regex.
On how to filter, I can send any address straight to the trash apparently just like simplelogin. I’ll know who sold or leaked my info because it’s in Bitwarden and I can just search my vault to see who I handed that particular address to.
This is why I just moved from protonmail to Fastmail. With Fastmail I can send from arbitrary addresses using my domain. Why it’s not that simple with proton is beyond me and now that I’ve tested everything with Fastmail these past few weeks, I see it’s a choice.
I almost signed up for simplelogin but realized I was being sold something that should just be included. Plus setup was convoluted as fuck.