Really easy, and costs me very little to do (CloudFlare is my registrar; Fastmail my email host). Highly recommend you buy a domain for email. Looks professional, too.
The way around this is that they let you do alias+something@domain.com, or for sites that won't accept a plus address, alias@something.domain.com. This works very well for me, although I get funny looks when some vendor asks me for an email address and I give them e.g. alias@your_company.domain.com. Especially fun when you end up having to spell it out while the clerk eyeballs you like you're trying to screw with them.
I started doing this thinking it would let me identify who was selling/leaking emails to spammers. I now have a lot of unique email addresses like this, one per vendor. Fastmail is either excellent at filtering spam without me ever seeing it, or vendors just aren't selling/leaking my address(es); the only 'vendor' that I'm sure distributed my address after 3+ years is a state government entity.
It looks like all tiers can do 600 + 15 per user; I don't remember if it's always been like this, but I was pretty sure there was a limit.
Looks like a subtle difference between alias, with its outbound identity, and catchall inbound.
This was a great feature of Fastmail but it isn't portable as many other mail providers don't support it. So if you think you might want to switch in the future, keep it in mind, the + addressing is easier to migrate to other providers.
An Njal.la hosted domain was successfully transferred to Namecheap (through no fault of Njal.la). Obviously, this is very rare and extremley targeted.
[1] https://registrarsec.com/ [2] https://www.vice.com/en/article/qj8833/dark-fail-fake-court-...