Replying to mail that arrived through masked email addresses will also reply with the masked email address and not accidentally leak your main address.

Also creating masked mail addresses in the first place.

On the down side it's an Electron app, on the upside it's JMAP.

I use Mail on macOS, but occasionally the Fastmail web app in a browser. Here are a bunch of things I use the web app for besides configuration stuff:

- Block sender (whole domain)

- Report phishing

- Mark as spam

Because that stuff should be done on the server, not on the client and if you mark stuff as spam in Mail, it's on the client afaik.

Marking as spam in mail.app moves to junk folder on server and triggers server side report as far as I know.

Almost every mail client I've used doesn't handle (catch all) aliases/identities properly including responding with the correct address. Also much better search.

I would hope it uses JMAP, not IMAP

I think it does use JMAP because that's how the core Fastmail web experience works.

Why is this important to a user?

Mostly performance benefits that JMAP enables, and masked email addresses feature for example. - https://www.fastmail.com/blog/how-and-why-we-built-masked-em...