Exchange mail flow rule to use connector when no mailbox found

I think my scenario is a hybrid scenario, in that I have MX records pointing to Exchange 365, and all the internal company emails are in Exchange 365. However, we also have a couple of hundred other email which are simply redirects. Instead of moving them to 365, I want to leave them where they are, on a hosted MailEnable server.

Although I have got it working, it feel like my rule could be a lot easier.

My current rule is:

• If recipient's domain is "mydomain.com"
• Use the following connector: "redirect all mail to external server which also uses mydomain.com"
• Except if: The recipient is: "list of 20 or so real email accounts"

The problem with this clumsy setup is that I must amend the list when I add a new user. Not the end of the world, but one more thing to forget.

I would prefer:

• The recipient is located "outside the organisation"
• Use the following connector "redirect all mail to external server which also uses mydomain.com"

The problem is the definition of what is considered "outside of the organisation". They are both using the same domain, which indicates both are inside, but my Exchange365 organisation has its Accepted Domain set as "Internal Relay", which AFAIK should allow what I want. Except it isn't.

What am I missing here? Or am I taking the wrong approach completely?

You could create a distribution list which includes exception users, if you do this, you only need to add new users into this distribution list not modify the mail flow rule.