Exchange on premise

I would like to know the best possible solution to block outlook connections (i.e. autodiscover, mail transport....) to only use specific mailbox servers. Here's my scenario. My root domain is called Philly, where I have three exchange servers ex1, ex2, and ex3 installed. Each of the exchange servers are configured with hub/cas/mbx roles. I also have 3 Tree domains named sales.com, HR.com, and marketing.com. I would like to have users from each tree domain connect to the respective exchange server where their mailbox is created when utilizing outlook. Example, mailbox users from sales.com domain will only use ex1, HR.com will use ex2, and marketing will use ex3. I have configured mail flow policies to prevent mail from being sent to other tree domains. Users from each domain will only be allowed to send to users within it's domain. I have also created Address book policy routing agents so users will only be able to see users within their respective domains address book. They will not be able to see the GAL. All of the above is working fine, but I'm running into a problem with Outlook connecting to any of the Exchange servers. How do I restrict Outlook to only connecting to a specific mail server? Is there an exchange powershell command that would restrict connections ? Thanks any help is appreciated.