I have a couple domains that are very low volume for outgoing mail. I use Migadu. I’m happy with their cheapest tier ($19/year for both domains). They have catch-alls and many other nice features.
Edit: They have no hard limits on the number of addresses, users, or domains and such. They just want you to be reasonable. You choose a tier based on your average quantity of outgoing mails per day. Again, there are no hard limits; they won’t cut you off unless you abuse the system.