Migadu has a similar offering (sans the R&D on the JMAP but contribute to https://git.sr.ht/~migadu/alps) but a more reasonable (ie consumption-based) pricing [1] for people who own multiple domains. Just a happy customer.
Migadu dropped its free plan last year in the middle of the pandemic with just one month’s notice to customers. Businesses do need to make money, but this sudden decision with a month's notice during a perilous time was customer hostile. I can’t recommend it just for this reason.
Reminds me of EndJunk.com that I had almost all of my email through. They were just a catch-all forwarder. Went under one day with zero notice and I lost access to all those addresses. Some services let you recover an account if you remember the password, others make you confirm an email change at the old and new address. I lost those accounts. I’d have been happy to pay for the service instead of them going under.
Did they ever have a free plan? I guess not with custom domains. I also know they are a small Swiss company, so the pandemic may have put their existence on the line if they had a surge of free signups.