Fastmail may offer the mail service I need. I am not happy about a couple of things on the pricing page, though.

I need to be able to use the service to provide email for a minimum of four other users over at least four TLDs; eventually, I think, I’ll lapse some of the domains and probably only keep one.

So a “5 alias” limit does not meet my needs, I think.

Additionally, they provide TLD-addressed email only as a premium option even for the highest-level account: it’s a dollar a month per domain with ten-cent fee per address. This may be negotiable.

Looking at it, it’s as though they’ve designed the pricing structure not so much against the needs of someone to operate a domain in the manner that, say, a business needs to use email, as around it.