You can use x3 domains with catch-all. Just add each one as "@domain.tld" and setup mx/spf/dkim/dmarc as usual. Then the domain will receive with catch-all. How

You can use x3 domains with catch-all. Just add each one as "@domain.tld" and setup mx/spf/dkim/dmarc as usual. Then the domain will receive with catch-all. However, prepare to be spammed, as many spammers figure that "mail@domain.tld" are always available, so that one will frequently receive spams. If you need more than three domains, try Migadu(not affiliated, just happy customer), they have no formal limits to their "micro" plan and is cheaper than FastMail. Migadu also allows adding alias domains(something I haven't seen anywhere), basically if you have a mailbox like "merlinscholz@domain.tld" you can attach some more domains as alias, like "@domain2.tld" "@domainx.tld" and those will all receive/send/operate as the same "merlinscholz@domain.tld". Neat feature I haven't found yet on other services.