Reply all doesn't cc my wife if we both have email accounts in the mail handler

> The fact that this has been a topic of discussion for over ten years means it is a use case that catches people.

Maybe in your circle, but I can honestly say this is the first time I've ever heard of this.

> Whether the email addresses are for different people, or the same person is irrelevant

We agree here. The issue, though, is that Mail.app has no idea of whom each email address relates to. How does Mail.app know whether any given account is 'you' or not? (for that matter, it doesn't even know who 'you' are in the first place).

> If fact, Apple must have specifically written a "use-case" to prevent this, rather than one which would allow it.

My guess is that it's specific code to avoid a recursive loop that repeatedly sends mail to itself, potentially overwhelming either the mail client or the server.

> Please just delete the offending code or pass me to a manager who can justify it.

There is literally no one here can do do either of those things. We're just regular users like yourself.

The official channel to request new features or changes, is https://apple.com/feedback/

I'm guessing that Mail.app is scrubbing the list of recipients.

Mail.app has no idea that you@yourdomain.com and wife@yourdomain.com are different people - as far as it's concerned they are just two email addresses, presumably for the same person.

On that basis, if you 'reply to all', there is little need to send yourself two copies of the email, so it 'conveniently' removes one of them.

I think this comes down to your unique use case.

I don't see any way of changing this within Mail.app. It's just not a use case that Apple planned for. The only workaround that comes to mind is to use separate mail applications for each account.