Yes it's trivial, but I'm asking for a commercial provider one can trust that uses software that doesn't suck horribly and one that doesn't just cut you off immediately if your payment to them fails, or locks your account until you phone them if you try the password too many times etc.

If I wanted to host my own I wouldn't ask about providers...

Email these days is about much more than just email. Calendar integration and sync between devices that lets you access email on said devices as well as resiliency one would want for an essential service mean just "hosting your own" is definitely not trivial.

Then there is antispam etc.

For me, it’s cheaper and easier to pay someone else