You can login to the SimpleLogin.io interface and setup reverse aliases even if you create them in proton pass. They don't have the integration fully down yet so it's clunky but it works.

Were you using simplelogin.io, which is part of Proton? It is actually possible to reply or send mails from the aliases you create there. The feature is called reverse-alias.

SimpleLogin does allow you to send emails from your alias, though

If you’re talking about a SimpleLogin, you CAN email from it. I forget the exact terminology (I can check if you need) but you can generate a forward address that you email and the receiver (Spotify) will see it as If you have emailed from your forwarding address.

Can’t you make a reverse alias through the SimpleLogin ui? Since that’s what proton pass uses for the aliases.

If you use fastmail and a custom domain it automagically lets you reply to email from the email they sent you. i.e. if you have COMPANYA@mydomain.com and you reply to one of their emails it comes from COMPANYA@mydomain.com

Most paid email providers let you setup a custom domain with a catch all address, its worth it. Not all have this handy reply email feature, and you have to configure a custom response address, annoying it but works too