SymptomsMails are not get delivered to distribution lists containing external recepients. You can see that messages are stuck in 'Messages Awaiting Directory Lookup' queue on the SMTP Front-end.
CauseIf you disable the Exchange Information Store on front-end servers, messages remain in the 'Messages Awaiting Directory Lookup' queue.
ResolutionIt is recommended that you keep the Exchange Information Store service running on Front-end servers to process messages successfully.
Note, if the Front-end server does not serve as a Transport Gateway, then the store is not required.