Remove a domain from Exchange 2007

I have a sub-tenenat, CLF, that has shared my Exchange 2007 Server for some time.  They are now moving out and I have migrated their email to a hosted Exchange account and changed their MX records.  They are able to send and receive emails on that new server just fine with the exception of emails to and from my Exchange server (their old one).  I know I have got to be missing something but can't figure it out.  

Once I verified their new email was working properly, I have performed the following tasks on my Exchange server:

 •Removed their domain from Organization Configuration > Hub Transport > Accepted Domains
•Removed the email addresses from individual accounts

Is there anything else I need to do?
Who is Participating?
tmartin40Connect With a Mentor Author Commented:
All external MX records are pointing to their new one.  I do not maintain DNS records for their domain.  I found the problem... We use a service, Mimecast, that was rejecting all emails between the two of us.  They have resolved the problem.
Nick RhodeConnect With a Mentor IT DirectorCommented:
Did you perhaps have any DNS Zones setup for their domain?  Are all external MX records and DNS records adjusted to point to their new exchange?
tmartin40Author Commented:
Further investigation (after following NRhode's advise) found that MX records were correct but Mimecast was blocking the trnasmission.
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