Unconfigure Exchange from accepting mail for multiple domains

Unconfigure Exchange from accepting mail for multiple domains

For some time my company (A) hosted the email (Exchange 2010) for another
company (B).
Company B has moved their email to another hosted solution. That went
smooth. They are getting their email on the new system. Their mailboxes
are still on our server but mail isn't being delivered there so those
mailboxes are just sitting there. (until I do something with them)
However whenever I try to send an email from A.com to B.com it is being
delivered to the mailbox hosted on my Exchange server and not the new
server they're using. I've removed B.com as an Accepted Domain and the
associated E-mail Address Policies for B.com. What am I missing? Have I
done something wrong?
How do I make it so I can send mail from A.com to B.com and it arrive in
B.com's users' mailbox on their new setup? Thanks.