There is no email (the protocol) in iMessage (the app). You can use somebody's email address as the recipient for an iMessage (the protocol). No email is ever sent.
I don't remember if MMS is enabled by default in iOS but theres a toggle to disable it, and realistically there's very minimal real world use-case for MMS these days.
It's still not sending emails, though. The iPhone Messages app sends SMS, MMS, and iMessage; email is the responsibility of the Mail app.
In an MMS, it could be unclear, but only if you choose to put an email address in the “to” field. If you know it is an MMS, and you only use phone numbers, then it will not be an email.