nostr:npub1c4sgd346sl9uh7mjhn99xjsu7t8dwm54kl6hxp3k0qyl4kz6v94qh5aqv4 
Not as far as I know.
I use mailbox.org too and I couldn't find something, I would love to use their cloud but alas.
Speaking of, how do you use your custom domain?