If you use your own domain (as the parent suggests), the cost is creating an account with any other e-mail provider that supports custom domains and updating DNS entries.

Since he uses his own domain, he can move elsewhere if there's a problem with Google. I do that too, and it's indeed much simpler than self-hosting.

On that note, I have amazon manage my custom domain.

Now I'm terrified that I'll return one too many pairs of socks or something, and get the retail consumer part of my account banned (we buy a lot of stuff on Amazon).

Does anyone have experience with what happens to AWS resources (specifically, domains) when that happens?

It sounded like he's actually keeping the emails on google, even though they come on a custom domain. And that doesn't seem safe to me.

I use a client, so have a local IMAP backup, and regularly back this up to my NAS.

That's not really my worry, though. What scares me about being heavily invested in blah@gmail.com or blah@notmydomain.com is lockout from services. As long as I have my own domain then I can just switch out the DNS and I'll still be able to get into whatever web service I signed up to using my email address.

For backup he could get Thunderbird or similar to fetch the emails are regular interval. I think it's generally a good idea to do so.

The same thing camps happen with any company. If you have your own domain you can get a new email host, change your mx record. Update your email client to use the new credentials. If you use mobile email, upload existing recent email from your desktop or restore/ import from backup. This means not registering the domain with your email host to avoid problematic delays