I couldn't find this documented anywhere but I feel it's a pattern that others might want to copy. I have a partner and we wanted a way to have a shared email account. We also wanted to be able to use anything@customdomain.com and be able to reply from that address created on the fly. We also wanted to have our own custom sub domain each (anything@person1.customdomain.com).
This is mostly possible with Proton and I thought it would be worth explaining how:
My partner does not have a Proton account yet so in order to implement the full setup we would need to create an additional account and pay for it, so you'd probably need "family" over "duo".
Create the following proton accounts:
Now make the shared account the admin and sign up for whatever pricing plan you use.
We used a password manager so both of us can log in to this account. (But Proton if you are listening it would be nice to have an official "shared account" way of doing this).
Now create the other two accounts.
Now you need to create your domain on whatever DNS provider you use.
Now go to simplelogin.io and sign in using the shared proton account (No additional account needed, it uses the proton authentication).
Go to Domains in simplelogin and add a new domain (customdomain.com). Now you need to follow the steps to register the DNS with simplelogin: verify ownership, MX record setup, SPF, DKIM, DMARC. Once all of those are working you should be able to send and receive email using that domain.
Now to set up the other accounts, log out of simplelogin, then log back in but log in with the other proton account (person1). You may have to log out of proton and switch accounts there.
Once you are logged in as person1, you will need to setup the DNS for the subdomain: person1.customdomain.com and do the same as previously.
And do the same for person2.
Now you have a shared account (which you share a password for) and a personal account each that you don't share the password for.
You can now create email addresses on the fly under all the accounts:
Sign up to a website using anything@customdomain.com and once an email is received it will appear in simplelogin as an Alias. If you later want to block that website, just go to simplelogin and turn off the alias.
You can also now reply from anything@customdomain.com, however it's a bit of a faff:
If the Alias already exists, in order to reply from an Alias you need to go to simplelogin and click "Contacts" to add a "reverse alias". Once you have it will have an option "Copy reverse-alias". Click this and you will have an email something like: "Company | hello at octopus.energy" <hello_at_octopus_energy_randomletters@simplelogin.co> if the Alias doesn't exist yet you will need to create it.
When you send an email to this simplelogin.co address, the simplelogin server will edit the email to make it appear as though it was sent from anything@customdomain.com
It's a bit of a pain but does seem to work. (Again Proton if you are listening, integrating this in to the email client would be great)
Anyway, hope this helps someone else :)