amazingefren I currently use two domains. There are tons of use cases to have.
My hope is that they don't severely limit the number of custom domains. Proton Mail only permits 3 custom domains on their unlimited plan which is frankly laughable for something named "Unlimited". Ditto their 15 alias limit, for such an expensive plan, the limits are ridiculous.
I have 8 domains which I receive e-mail for, which might seem incredibly excessive, but it was due to being able to pick up nicer variations of my name over time. It's sensible to have some type of limit but please don't be stingy like Proton Mail.
Additionally, users should really be encouraged to onboard with a custom domain to assuage fears of "What happens if Kagi Mail disappears in a year?" Since then they remain in control of their e-mail addresses and it really doesn't matter as much if the product ultimately fails.
I'd be curious to see how this pans out, though personally I'm pessimistic of the chances of success. There's a lot of competition in the e-mail provider space, vs something like search engines where Kagi has a much easier time standing out. I'd much rather see the time and effort invested in really fleshing out Kagi Assistant personally, as that tool is incredibly useful but feels barebones at present.
