I’m actually quite pleased with what I’ve ended up with now. One of the features of the Postfix / Dovecot server I’m running now is that I have recipient delimiters - for example, if my email is hiding@aussie.zone, I can sign up for Mastodon with hiding+mastodon@aussie.zone, and everything they send me will be automatically filed into the Mastodon folder in my account. Additionally, I know exactly who is selling my data this way. It’s a great system and avoids the unfortunate predicament you currently find yourself in!

That’s the best part of Proton+SimpleLogin. I use a “different email” for every account for which I sign up. This allows me to know exactly who sells my info and who doesn’t, by which address starts to get spammed.