Zoho isn't Google-size, but it isn't irrelevant, either. Sending mail from a self-hosted email server is far harder since the big providers might put it in spam or drop it even earlier.
> running your own mail server is the only way to ensure your email is not read by someone else
But any mail you send to someone else probably ends up read by Google/Microsoft anyway, since that's where their mailbox is.
Also, email security is a joke. It's 2020, and even TLS encrypted SMTP connections tend not to check for a valid certificate, making them trivial to MITM.
Practically speaking how does one MITM an SMTP connection? For example, from Google to Microsoft. They connect directly to the IP addresses they get from MX records + lookup. What's the actual threat vector/execution here?