Office 365 - rule to reject emails sent to one domain

We`ve recently installed office 365 and taken the opportunity to change domain name from xxxp.co.uk to xxxa.co.uk.  All users are the same so have changed to John.Smith@xxxa.co.uk.  I`m having an issue with some users still sending emails to John.Smith@xxxp.co.uk.  I`d like to either reject that via the OA365 Exchange or at a user level but struggling with concept.  Any pointers gratefully received.

Kevin
RS4KEVAsked:
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Vasil Michev (MVP)Commented:
Just use the "The recipient domain is" condition? And select the relevant action.

If you want to apply this to only a subset of users from said domain, add them to a DG and use the 'the recipient is a member of' condition
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