if a customer email's us outside of the netsuite system. how can we move that email into the NS system?

Have a look at Celigo's outlook integration (haven't used it myself) -https://www.cloudextend.io/netsuite/outlook-integration/p/outlook-for-netsuite/

Starting at free for limited use too. https://celigo.wistia.com/medias/ixo2xa8y7r

you can capture it as a Case.