Email Sending Restriction in Exchange

I need a solution that blocks certain users in my exchange organization from sending email to specific email addresses based on which email address the user is sending from.

Our Exchange server (exchange 2010) hosts email for multiple domains and each user is configured with multiple email accounts in Outlook.  let's use the following domains for example:

Domain 1: abc.com
Domain 2: xyz.com
User:  UserA

Each user has an email account in Outlook corresponding to a mailbox for each domain whereby they can send email from either domain depending on which clients they need to communicate with.
 
UserA can send an email to someone@specificdomain.com from their UserA@abc.com address;

but UserA should not be able to send email to someone@specificdomain.com from their UserA@xyz.com address.

we have two "identities" for our business and are trying to prevent inadvertent correspondence to clients from the wrong email domain.

all help is here greatly appreciated.