Decommision an email domain and have an ndr sent

I am using a Sophos Firewall and Exchange 2010 Server and I am discontinuing a domain and want an ndr/reject message  to  be sent to  the sender that the e-mail is no longer valid.
I am not sure where to make this happen.. do I discontune domain in Sophos have exchange send the NDr or what are the rules of the internet to where the message will be rejected.
When I delete the domain in sophos I get a message bounce back that says relaying not allowed.
I want the sender to get an email not valid message.
Thanks
kcassoneAsked:
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Will SzymkowskiConnect With a Mentor Senior Solution ArchitectCommented:
When you decommission your MX records on your external DNS when people try to send you email they will automatically get a bounce back stating the email/domain does not exist. No need to send out a NDR notice. As soon as they try to send mail they will be notified.

Will.
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