Hello Adan,
I would suggest to do the following:
access-list MSN permit ip x.x.x.x(internal subnet) any
class-map MSN
match access-list MSN
policy-map type inspect im MSN
match protocol msn-in
drop-connection
Policy-map global_policy
class MSN
inspect im MSN
Regards,
Julio
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Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC