He's an hardware engineer, not a software engineer.

After many years of self-hosting an email server I also came to the realization that it's not worth the hassle (in terms of maintenance / problems) and I switched to Proton Mail.

Using a custom domain with Proton Mail is not free (and all the other alternatives I know of are the same) and there's little advantage other than having a nice name. I think for a HW Engineer this is good enough.