Windows

Let me see if I understand company has domain1.com and domain2.com, user only has user@domain1.com, is that it? So any email comming in for user@doamin2.com should be blocked....I don't know about blocking it, but it should cause a NDR to be sent.If the user is recieving the email with the other domain you should check the user properties email address tab. For each doamin name that is active Exchange automatically creates an email address for each user, if this is the case just delet the unwanted address from the user properties and then email to that address will bounce a NDR to the sender.

You need to configure multiple recipient policies. Make sure that the recipient policy applies on all objects that need an email address with this domain (by using a filter) and possibly add additional addresses manually.
The if I understand your question, when a particular user falls outside the scope of a recipient policy, the email will be blocked because the recipient policy will not apply to his/her user object.

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