Will MX lookup ignore MX on server which also has NS delegation.

[Robin Farrell]

We have a client that is using NS to delegate their DNS to us and we will
host their MX records.
They are refusing to remove legacy MX and A records for the domain in
question from their DNS, yet they have the correct NS records in place for
the domain in question to be redirected to us.
Could this possibly effect how dns MX/A lookups are redirected to our
servers?