I have a few email addresses in use for different purposes and contexts, some of them are more than a decade old, and I don’t think I can close them up completely.
For example, I have an email on a server of an NGO I am working for, alongside my personal gmail used for Google services and an old personal yahoo, and a couple addresses associated with some apps that I’ve authored.
I’d like to use some of them alongside Proton email addresses. Since I can’t add proton to Mail.app, I need another mail client where I can have my proton email alongside other mail addresses.